The player puts the first finger in its place in first position, then moves the thumb and hand back a half-step so that the first finger on the A-string goes from B to A-sharp (or, enharmonically respelled, B-flat); in the second case, the half position could be viewed as a lowered first position.^[1]

Exercises in Half Position

 

Example in Half Position

 

It goes without saying that in half position, extended position is not possible, because the first finger is already positioned at the nut and therefore does not move back.