If we use the forearm alone when playing in the upper half, the bow stroke will be crooked.
The upper arm should come forward slightly at the end of the down-bow and back at the beginning of the up-bow.
Stroke your hand along a broomstick to mimic a whole-bow stroke and you will notice how the upper arm naturally moves in and out.
In the upper-half détaché the upper arm is always passive, that is, it responds to the forearm action.
This action is mainly used when playing at a slow or medium tempo, it becomes cumbersome at fast speeds.
This action is more apparent in those players with shorter arms.