What are upbeats?
Up-Beats, incomplete measures at the beginning of a piece.
How does that work and what is the (unwritten) rule for violinists
An upbeat is the last beat in a bar which directly leads to the 1st beat in the next bar, the downbeat.
A special one, which we will discuss here, you can sometimes find at the beginning of a score.
The very first bar is incomplete.
In the thumbnail of the video you can see the first note, and right after that a bar-line.
This example shows a 3/4 time signature, so there should be 3 quarters per bar.
The first bar only has 1 quarter note, this is what we call the upbeat.
How to play upbeats
How do violinists usually play upbeats?
With an up-bow, see the up-bow symbol in the thumbnail.
The next note, the first in the new bar is the downbeat.
We would usually play that with a down-bow.
The downbeat is the "heavy" beat.
Bowing downwards is naturally heavier than bowing upwards.
So, playing as natural as possible, play the downbeat with a down-bow, and the upbeat with an up-bow.
The video on upbeats
Let me explain it all in detail in this video.