Rage eruption is a SLASH'EM technique exclusive to dwarves. It is similar to power surge, albeit it affects the HP. It increases your current and max hp by 50 + (4 * TECHLEVEL) and decreases them in every turn by one (this will never kill you). The technique lasts for 50 + (4 * TECHLEVEL) turns, after which your max hp is set to its previous value.
This technique is unavailable if you are polymorphed.
Although this technique is very powerful and saves many dwarvish lives it has to be used with care- you will end up with 1 hp that cannot be healed, so it is wise to run upstairs or engrave Elbereth after the main problem is taken care of. Also, be careful around koalas. Their otherwise harmless touch will abort this technique instantly, stripping you of all your extra HP in the middle of battle.