The bow-change action is important for many strokes (fast détaché, forearm sautillé). It is particularly useful when playing at the heel of the bow, although it should be accessible in any part of the bow.
A reluctance to play in the lower half of the bow can be caused by a tight bow hold or a faulty bow-change action at the frog.
Variables in the bow-change action include:
Releasing bow speed.
Releasing bow pressure.
Lifting the weight of the bow.
Or a combination of all three variables.
Usually the action involves a passive bending of fingers and thumb at the moment of change to the down-bow. There is often also a release of the 1st finger, especially at the first pad near the fingernail.
With slower strokes, the fingers can be more active during the stroke before the change, that is they begin to move early, and fingers and thumb curve at the end of the up-bow. This finger action is similar to a collé action.
Beginning Clicks - Introduction to Collé