It’s not that large of an incursion force, I think something like 2k of some of their better troops who would normally be acting as a quick ground reaction force. So they aren’t really moving people off of the front line, though they may have lost the ability to quickly reinforce one front or another.

From what I’ve seen, the salient is being used to probe the strength of the Russian’s western flank. It doesn’t appear that Russia was expecting an offensive, and didn’t have their own version of a quick reaction force held in reserve.

Unless the Russians can move men and more importantly artillery to the area, there’s a risk the salient could be used to roll their western flank, cutting of their border guards from their supply lines.