I held onto the promise of a forthcoming explanation (and read ahead in the book, anyhow). Honestly, someone who can't deal with a little ambiguity when starting out on a new technology isn't going to last long in the field anyhow, especially with the amount of self-learning that you end up doing.
Agreed, for a number of students the promise to explain it later will suffice. Others - when given magic boilerplate that does strange, intended things - will recognize it for the black magic it is and will want to wield it as well.
They will dissect, examine and research it on their own and their learning will be much deeper.
Well, that's me in a nutshell though. If I don't get the answers I need soon, I get frustrated and it becomes nearly impossible to stay focused or motivated. With web development, I was lucky in that I had teachers that were willing to humor my curiosity, and I could also hop on the internet and IRC channels.
Self learning is the opposite of the classroom issues I had. I can start a webpack tutorial, it'll mention Make, and I can click links and chase down information, and just keep my stupidly ADHD brain entertained with more and more new new new things. This is how I have to learn things, apparently.
That sounds more like a problem with incoming student's expectations than with the teaching method. Almost every interesting (i.e., sufficiently complex) subject isn't arranged in a pedagogical hierarchy. You can't explain history from the first day, because you'd have to explain where that knowledge came from. You can't explain math without pushing things for later because students are not prepared for generality on day 1. You can't explain human biology by describing various cells. Etc..
If a student is going to 'check out' because they didn't get a question answered, then they're probably just going to fail. See your teacher after class, ask a friend, look it up online... If are a 'curious student' doesn't see these very obvious methods of satisfying their curiosity, then they're probably not worth a teacher's efforts.
For a particular kind of curious student, "just do it because you have to" is a very fast way to them checking out and doing nothing at all.