You know when your forearms feel like they are going to burst open and you can't physically close your hand? How about when you get the elvis leg and start shaking all over? Or, better, when you go for the next hold and think "I'M SO PUMPED?" Well, by then you are probably off the wall.
The truth is, endurance is very mental. Sure, climbing laps and doing routes back to back, circuiting, and eating healthy helps, but in all honesty, that can be hard to do sometimes. So what do you do when you need an alternative method of holding on forever?