My 2 simple rules for crunch mode:
1. Manager better be there. Me personally, i would dive in and actually start doing code reviews. But even if I had absolutely nothing to add, I had to be there to show support. Even for window dressing. You call for crunch mode, you better be present. Even if its just to buy lunch or starbucks for the devs.
2. Coding is like breathing. Crunch mode inhale must be followed by re-tooling, decompressing exhale. You can only inhale so much.