Finger substitution
Changing which finger holds a note while it's still sounding, with no new attack and no audible bump. It's a small, almost invisible move — used to free up a finger for what comes next, or to set up a smoother shift a beat later.
The same note, two fingers
How to play it
Keep the bow moving evenly through the whole note — the substitution happens entirely in the left hand and should be inaudible if it's done well. Practise it slowly enough to hear whether a bump sneaks in.
Why it's harder under the bow
In pizzicato, a finger substitution is nearly free — the string isn't ringing continuously the way a bowed note does, so any small noise from the swap gets masked by the pluck's own decay. Under the bow, the note is sustained and exposed the whole time, so any bump, scratch, or pitch wobble during the swap is fully audible. That's what makes this genuinely harder than it looks on paper.
Try it
Try
Play a long, held D-string E with your 1st finger. Partway through — bow still moving evenly — silently bring your 2nd finger down onto the exact same spot and lift the 1st finger away. Listen closely for any change in pitch or a click at the moment of the swap. Repeat until it's genuinely silent.
The usual mistake
Rushing the swap. A fast substitution is far more likely to bump the pitch or scrape the string than a slow, deliberate one — there's no reward for speed here, only for the swap being inaudible.