Depending on the circumstances, we may need to hold fingers down or lift them. We hold fingers down for the following reasons:
the finger will soon be used again
to add stability to the hand frame
to make a guide finger for a shift
This is how fingers are used in the one-octave scale starting on open A.
On the E-string leave 1st, 2nd, and 3rd fingers down as we are going to use them again immediately. Peel them off one by one for the descending passage on E.
When descending on the A-string we need to place the fingers independently as they are not already down.
As a general rule, hold fingers down as late as possible in ascending passages and make sure that the new finger is ready well ahead of time in descending passages.
A Major Scale - violin
D Major Scale - viola