It doesn’t impact other admitted schools at all. Here’s what happens:
You agree to accept a spot on Harvard’s waitlist. Then, by May 1st, you must also accept an offer from a college that admitted you. In most cases – aside from HYP – the accepted student must put down a deposit by May 1st at the college they have agreed to matriculate to.
Harvard won’t know how many spots are available for waitlisted students until AFTER May 1st. So you wait. In the past, there have been about three rounds of waitlisted notifications starting around mid-May and going until mid-June.
If you hear nothing from Harvard, you do nothing. If Harvard calls (yes, they actually telephone you, they don’t email waitlisted students), you will have about a week to consider Harvard’s offer. If you accept Harvard’s offer of acceptance from the waitlist, you must immediately let the college you agreed to matriculate to know you are withdrawing your acceptance. In most instances you will lose your deposit at the college you initially agreed to attend. That’s the entire process. The college you agree to matriculate to understands the waitlist process, as most likely they too have a waitlist.