Player development is a huge topic currently and a lot did change due to the statcast numbers. Many players have improved their swings and became starts out of nowhere. The most famous examples of this are probably Josh Donaldson, Jose Bautista and JD Martinez but also many other guys like Daniel Murphy and Justin Turner.
However when listening to broadcasts you will still hear many of the old cues or the broadcasters will speak in a derogative way of the what they call “launch angle swing”. They will call players selfish and not caring about Ks but of course the term launch angle swing is stupid because every swing has a launch angle and it has been proven that launch angles of about 10-30 degrees are ideal. https://community.fangraphs.com/evaluating-statcast-hit-type-boundaries/
And even for the softer hitters an angle of about 5-20 degrees is most productive, only balls at 20+ degrees are extremely exit velocity sensitive.
Now here are some cues we are hearing daily in broadcasts and also still by many coaches although there are more and more “new style” coaches even in pro ball like for example Jason Ochart at the Phillies or Jeff Albert at the Cardinals (formerly Astros).
Cue 1: Squish the bug
This is a quite common cue. The idea behind it is that the squishing motion (turning the leg inwards on the ball of the rear leg) drives the hip turn and allows the hitter to stay back.
However this is not really what we see with elite hitters. Elite hitters often have their back foot off the ground and their weight against the inside of the front leg. You don’t want to climb too much on the front leg and still stay behind the front side but the weight does transfer with the swing. Force plate measurements have shown up to like 1.5 times the body weight of force on the front foot.
Cue 2: Extension at contact
You often hear after a homer “he got full extension at this one”. A common teach is to extend the arms to move the knob of the bat to the ball while keeping the bat lagged and then like a whip throw the bat head forward.
But when watching pros this is not really what happens.
With many homers the arms are still bent a lot. On pitches middle in the arms often form a „box“ with both elbows bent and on outside pitches the front arm tends to be more extended but the back arm still bent.
The hands are not really pushed forward as the bat turns down in the zone.
You essentially want to turn the bat to the ball without pushing the knob forward and then extend through as you make contact reaching extension well after the ball has left the bat.
There are still hits with more extended arms but this is usually when the timing is early for example on an offspeed pitch. Because of that the deep contact with following extension works as a „timing reserve“ in case you are fooled by a pitch.
Generally the hands will maybe start slightly down but then level out quickly and go forward to slightly up (kind of like a “J” laying on the side)
Cue 3: Swing down
The shortest line between two points is a straight line, right? The bat starts higher than the head and makes contact below the waist. This means the shortest path is a diagonal line down.
However that is not what we see with the pros. Their bat turns down like a „C“ behind the body and then goes slightly up. This path is longer but since the pitch comes in a downward plane the slightly upward path means your bat is on plane longer and can hit the ball even if timing is off. This is not a new concept, Ted Williams described that in his book the science of hitting but this approach was replaced by the more linear direct path in the 70s and 80s in teaching and yet it is not what the best do so Williams was right. The best hitters in the game have a positive attack angle of like 10-15 degrees. More uppercut can help power production but might cause some more swing and misses while a lower attack angle of like 8-10 degrees might be ideal for conistency but not ideal for power output.
Early “online hitting guru” Jack Mankin already noticed that in the late 1990s
Cue 4: Don’t drop the back shoulder
Traditional teaching says don’t drop the back shoulder and make contact with level shoulders and a level bat.
This is actually kinda correct on the high pitch at the letters. If the pitch is high dropping the bat too much indeed can be a flaw and this makes some good low ball hitters struggle against high heat. However on low pitches the back shoulder and bat head should actually drop. What you can see is that the bat is always about perpendicular to the spine (level to the top of the shoulders) and this whole axis tilts like a clock hand through the zone.
BTW even on high pitches the bat is usually not completely level. On high pitches the vertical barrel angle usually is at around negative 15-20 degrees and like 45 degrees on very low ones (numbers from DK Williardson’s book „Quantitative Hitting“.
Only on extremely high pitches you will see a level bat.
Players with a too flat vertical bat angle on lower pitches will often have trouble lifting pitches in the bottom half of the zone especially to the pull side. I talked about this in https://community.fangraphs.com/finding-keys-to-elevate-the-ball-more/ article and analysed Arenado’s swing in that regard. https://community.fangraphs.com/how-nolan-arenado-avoids-the-ground/
Cue 5: Keep the lead shoulder in
It is true that the lead shoulder shouldn’t rotate before foot plant. You actually want to keep the shoulders still while you start to open the hips into foot plant to create some separation but then the shoulders do turn well before contact.
Studies have shown that hip turn speed peaks slightly before trunk turn speed. Both peak relatively early in the swing (about 60 ms before contact with the swing taking 150m ms) and then slightly decelerate to allow the bat to accelerate past the trunk. This is called the kinetic chain (Forthenbaugh 2011).
Cue 6: Hit the ball on the ground
Many coaches like to teach backside grounders because the fielders need to make a play and youth hitters don’t have the strength to hit homers. However at the highest level launch angles of 10-30 degrees are the best. Those guys are strong but even for low exit velocities angles of 5-20 degrees play best.
Here are angles marked in a cage:
We usually teach hit the L-Screen waist high in the cage and for preteen kids I do teach that too but for 12 and older kids I think it is ideal to hit the ball over the L-Screen because that zero degree cage liner is going to be a grounder in a game due to gravity pulling it down.
I’m not advocating swinging for homers but I think the goal should be to hit high liners with an apex height of like 15-30 feet (5-10m) at least for advanced youth players. Those hits have a very high chance to b e a single (over .500 BABIP) but also a good chance to be a double. Teaching to hit high liners thus is a good compromise between the „launch angle“ teaching and traditional hit for a base approaches.
Cue 7: Stay back
Before you swing you need to load the body up to generate maximum force. Many teach to go back and then forward. Some players indeed go back but it is important that the actual load occurs on the forward move. The head is not really staying back (see point #8) but after loading on the back hip it indeed goes forward and slightly down, but the weight tries to stay in the back hip until just befoe foot plant. While this forward move happens the body very slightly counter rotates and the hands go back some. It is very important that the hands load on the forward move and not while going back so the body can create stretch between the lower and upper body. Here is a good video of the load while going forward.
— Kenny Collins (@kcbases) May 16, 2019
Staying back means keeping the weight on the back foot and keep the head still to wait better on soft stuff. However if you watch almost any pro hitter the head will actually move forward (and slightly down) but once the front heel is planted the head doesn’t move.
However while the head does move forward in relation to the ground it shouldn’t move ahead of the hips. The hips and trunk is very slightly coiling in and the head moves over the inside of the rear hip and stays there until foot plant. Some call this move forward yet back because the body is moving forward but with the rear hip and head going forward at the same rate. The body is essentially riding on the rear hip and then after the hips start to open the weight is transferred to the inside of the front foot.