Bat drag is by far the biggest issue we see with amateur baseball and softball players. By definition, bat drag is when the rear elbow drops prematurely during swing launch. You can see it when the player begins to rotate their upper half, if the rear elbow is in front of the knob there is bat drag (see left side of picture).
Bat drag leads to a couple of swing issues such as early barrel drop (which causes foul balls and pop ups to the opposite field side) as well as wrist rolling (if a player tries to over-correct the barrel drop due to their rear elbow getting stuck). Due to the abundance of players with bat drag, all of the Epstein Hitting Online Academy drills are designed to remove this issue from a player’s swing.