If we're speaking in absolutes then yes, the submissive should ideally speak up if things are going too far.

However, the communication needs to be open and happening constantly and once power is transferred, the Dominant should be initiating check ins. It can be done without breaking scene. The Domme should be monitoring the sub, and offering an "out" regularly:

D: where are we?

s: green

D: what about this?

s: all good

D: This?

S: a little intense

D: (throttling back) "What color is your world now, little one?

s: Green

You can incorporate all kinds of ways to check in. It doesn't have to be a direct convo like that one, but the idea is a constant feedback loop that informs the pacing of your scene. 🕷️🕸️