But this does not happen *before* you take the damage, it happens before you fall to/below 0.
Not technically. Technically, the damage never happens until after the Bear's Endurance. Otherwise, Bear's Endurance could not take a PC from bloodied to not bloodied before the damage occurs and prevent a monster effect that occurs due to the PC being bloodied.
An immediate interrupt lets you jump in when a certain trigger condition arises, acting before the trigger resolves. If an interrupt invalidates a triggering action, that action is lost. For example, an enemy makes a melee attack against you, but you use a power that lets you shift away as an immediate interrupt. If your enemy can no longer reach you, the enemy’s attack action is lost.
The sequence is:
1) Damage is calculated and player is informed.
2) Player determines that it would put his PC at zero or below, so he uses Bear's Endurance.
3) PC gets 25% more hit points (plus any other adds for magic).
4) Player subtracts damage from PC's new current hit point total.
Just because the player figures out that the damage would take him below zero does not mean that the damage happens first. For example, the DM might thrown in extra monster damage because the PC is bloodied and if Bear's Endurance brings the PC's hit point total above 50% and out of bloodied, that extra damage has to be subtracted.
This could even result in the PC not getting hit at all. Say that a monster has +1 to hit vs. bloodied foes and rolls the exact number needed to hit. PC is no longer bloodied and monster misses.
S'mon said:
Contrary to popular belief, Interrupts don't 'turn back time'
For all intents and purposes from the player's perspective, they do. The event is modified from how the dice indicated that it occurred.
From from the PC's perspective, the original event never happened at all. It's not that time is turned back for the PC, it's that the power prevented something from happening (or minimized an effect) that might have occurred. The PC doesn't necessarily know that the event actually got changed.
The player knows. It's like Doctor Who. The doctor knows what was supposed to happen, the rest of the world knows what ended up happening.