The best model I seen to explain this is the “leaky pool” model that Aaron Boster uses in this video: https://www.youtube.com/watch?v=fZPQ48N-nIs&t=154s
Basically even without a new lesion if your “reserve” drops you will expose existing symptoms. To prevent the leak you need to follow all the advice but the main things I watch for are sleep and stress. Of course diet and hydration are biggies and avoiding heat if you are prone to issues with that.
I’ve transitioned from RRMS my neuro feels and more into SPMS (Dx 15 years ago, now 55) and haven’t had new lesions in some time. Have moved onto Ocrevus for a DMT). I have more a return of prior symptoms instead of new ones so feel like I am always fighting the same battle 😉 but at least I know my enemy! I’m trying to take better care of myself and keep pushing off progression.