The mechanism is the Zeigarnik effect: unfinished tasks occupy attention more than finished ones. Ask a question and do not answer it, and the audience is holding something. Answer everything as you go and each moment is complete, which is comfortable and is the reason well-made explanatory content is so easy to stop watching.
Loops stack. Open one in the first seconds, open a second before closing the first, and close them in a different order — this is the entire structural technique behind most long-form video that holds attention past the point where the subject alone would. The discipline is that every loop must close. Loops left open read as a piece that wasted your time.
The commercial version is the reason a testimonial should not begin with the result. Stating the outcome first closes the only loop the piece had.
What is an open loop in content?
A question raised and deliberately left unanswered so the audience carries it forward. Unfinished questions hold attention in a way completed ones do not.
How many open loops should a video have?
More than one and all of them closed. Opening a second before closing the first, and closing them out of order, is what holds attention across a long piece. An unclosed loop reads as a piece that wasted the viewer’s time.