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.
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.