Belly Drum: The user slams its fists into its belly, mentally psyching itself up for battle. The user reaches a sort of battle frenzy, totally ignoring all defense in favor of crushing the opponent with highly-buffed attacks. This is the strongest attack buff in the game, but the user cannot even attempt to dodge or reduce damage from enemy attacks. Excellent accuracy.
There's no indication the user sacrifices HP, unlike Curse. From the description it seems to lower its defence to fully boost its attacks. It may also deal some damage to itself (3-5%) seeing as it 'slams its fists into its belly'. So it will, in short, take more damage from attacks in order to deal more damage itself.