Important information about bump stocks

[grayfox]

I don't own a bump stock, so therefore a question: which action or operation initiates the first round to fire? In a plain semi-auto it is the "trigger" finger pulling the trigger back that causes the firing sequence to initiate.
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For the bump stock, is it that same finger? or is it the forward press of the slide/bumpstock (I guess by the other, non-"standard" trigger hand) that initiates the first firing sequence? i.e, for a right-handed person, is the first round initiated by the left hand pulling the slide forward, or is it the right hand trigger finger somehow pulling the trigger?

Maybe someone who knows how these things work can answer.