Let’s let my favourite psycho scientist answer for me:
Really though, that tends to be best, jazz hands or ‘flashing’ your hands by closing and opening them repeatedly.
If you’re completely immobile and your hands are incapacitated then you can agree a single, or three loud sounds gets him to stop and ask if you’re safewording.
In that kind of bound and gagged scenario the dom should be really on high alert the whole time and checking in with you a lot anyway. For that kind of intense scene I do like the ‘green’ word too, which is a POSITIVE check in from him every so often…
e.g. Leans in and strokes your hair as your bottom recovers from a sound beating, and whispers, ‘Green?’ lovingly into your ear, and through the tears you nod to let him know it’s all good. Kinda thing.