Most problems hand you a surface deliverable almost immediately: the thing you were asked for, roughly shaped, technically true. The fast path is to polish that shape and ship it. Permission to descend is the practice of not doing that. It means treating the surface answer as a precondition rather than a conclusion, and going one level down to find what actually has to be true for it to hold.
The descent is not free. Every level down costs time that does not visibly touch the surface deliverable, so it needs a kind of permission, from a manager, a client, or just yourself, to look like you are not making progress while you are. What justifies it is the return trip. You come back up carrying something the surface pass never would have found, and the original deliverable gets sturdier for it.