It is the act of raising the elbow slightly when they actually pulling the trigger. I have witnessed it being done by lots of pros to the point that it led me to try and figure out what the purpose might be since it is not taught but tends to show up frequently.
My conclusion was they are letting gravity start the stroke when they let the elbow drop back into the starting position. The cue is started forward by the bump as you called it and then the biceps contracts to add power.
As to what is the benefit? I could see 2 answers. 1)You are not starting the forward stroke from a stopped position using a muscle contraction. 2)It works like a waggle and lends itself to getting into a rhythm.
It does add a timing variable and another moving part so it likely would not be something that would be taught. Instructors do their best to simplify the fundamentals and teach a stroke that is both accurate and repeatable. I would think the bump would be in the realm of advanced technique and could be both useful or harmful depending on the player.
Get the fundamentals down and then you can add your own personal adjustments or idiosyncrasies. Just never be surprised when one of them turns into a "bad" habit that you may have to go back to an instructor to fix.
