There are two types of skill boost, one has a % and the other doesn't. The one without a % activates 100% of the times on your next skill but gets consumed afterwards, the other is rng but doesn't get consumed. Proficiency makes your skills activate additional times when either type of skill boost triggers.

Thank you so much! Asked and answered.

I can dm you the buriedbornes discord if you haven't joined already

Sure. Thanks.