If you like to ride fast and like to ride in group rides that challenge you, then you know what it means to hit the wall. If you race bikes, you know what it means to hit the wall. It happens often enough and you know when you're there:
When suddenly alarm bells start going off in your head and you know that very soon you won't be able to hold the wheel in front of you.
FAST, OPEN ROADS
When the pace is high and unrelenting and you're still many miles from home, and suddenly you begin to realize you won't be able to hold onto the wheel in front of you for much longer.
When going in and out of corners you find yourself further and further back from the racer in front of you, and you realize you won't be able to pull yourself back to him many more times.
Most racers have experienced these in some form or fashion. You simply can't keep up with the pace, and you either give up, blow up, or you dig deep.
Digging deep is something that I don't believe many racers think to hard about - it simply is a thing you must do sometimes if you want to be competitive. But the first thing about it is obvious - you are in some state of low sugar energy, low oxygen, high lactate acid, high dehydration. Basically you're out of energy or your muscles are giving up.
The second thing aspect to digging deep is there is always a little bit of energy left in the human body. In nature, it's what would give you that last little bit of crazy strength to push off a bear or gnaw your arm off. It's a sort of reserve of energy essence or something. To get metaphysical, it feels like it comes from the heart area to me.
So I think when you're digging deep, you're essentially reaching into that crazy strength reserve that you're not really supposed to use unless, you know, a bear is attacking you. And that's what digging deep feels like. It's that crazy last ditch scramble to cling to life.
So if you're able to effectively dig deep you can sort of oscillate between crazy strength and regular, hard muscle use until the effort at hand has passed. Or you can use it for a limited effort that you know must be done, like clawing back the last 20ft to reach the back of an attacking peloton. It feels like madness.
I believe digging deep is not healthy for the human body - I think if you're really doing it (not simply trying really hard), it puts huge amounts of stress on all your systems. But it's a good tool to have in your racing toolbox if you know how and when to use it.