Why does this work?

My two-year-old sometimes has night terrors on days when she takes short naps (less than one hour).

My pediatrician recommended that we wake her up an hour after she falls asleep. This seems to work. But it’s not entirely foolproof.

Sometimes the night terrors come as early as 45 minutes into her falling asleep. Other times, we think we woke her up. But sometimes shaking her shoulder gently and saying her name isn’t enough.

I’ve figured out a new method that works pretty well. Instead of trying to wake her (which, at times, can seem pretty rude), I play some music about 45 minutes after she falls asleep.

Why does this work?
Why does waking her also work?

I suspect it has something to do with her REM cycle?