Good question.
Not 100% sure but from what I understand, when you get rocked, your brain is partly shut and is in need of oxygen for you to recover.
If you have good conditioning, you'll get more oxygen to your brain, quicker (partly because your blood will be more oxygenated, may have something to do with your heart rate also). Hence why they say having good conditioning improves your recovery time.
Hope it makes sense and I'm not too far off the truth
