Ask YC: When Y Combinator receives equity, when does it start diluting?

We've been offered a term sheet from a startup accelerator.

I think the terms state that their position will continue to adjust, and not be diluted, until just AFTER we raise a round greater than $250K.

Is this typical of accelerators? For example, does Y-Combinator continue to hold its full 2-10% investment just after a $1M round is raised?