Here's the trick:
You set up your macro to whatever HP number you want,
Then whenever you get attacked, the macro will automatically use the potion, when that happens, go to the inventory and click on the potion and put it down again. If your HP is still low enough, the macro will use the potion again, but without waiting the 1-3 seconds cooldown.
So everytime you feel that you won't be able to hold the monster/enemy, whenever your macro uses the pot, click on it (must be on the inventory) and put it back again, and it will automatically use it again without the cooldown.
This is easy to do, hard to spot that you're using this (most people think that you're just strong)
Have fun being undefeatable.
Edit: I have found another EXTREMELY good use to do this, try this with the Cure Potions and see what happens

. You will get rid of the debuffs in a matter of seconds!
It may be known in other sites, but this cheat/trick is a MUST for a site like this.