Where you direct your attention during a movement can affect how well you perform it. A large body of research on motor learning has compared an internal focus (on body parts) with an external focus (on the effect of the movement).
What the research generally shows:
An external focus, such as “push the ground away” or “throw towards the target”, often improves performance and learning compared with “extend your knees”
This has been found across many tasks, including jumping, throwing, balance and golf, although effects vary
Experts in particular may perform worse when they consciously focus on details of a well-learned movement
Simple, short cues tend to work better than long technical instructions during performance
Some examples of turning internal cues into external ones: in sprinting, “push the track behind you” rather than “extend your hip”; in a squat, “spread the floor” rather than “push your knees out”; in swimming, “pull the water past you” rather than “bend your elbow”.
The point is to describe the outcome, not the joint.