Whenever Bumi deals combat damage to a player, untap all lands you control. After this phase, there is an additional combat phase. Only land creatures can attack during that combat phase.
Bumi turns your lands into a battalion of beaters and then beats face with them. Earthbend lands into creatures, swing with Bumi for trample damage, untap your lands and take an extra combat where your animated lands pile on. Snowball land counters and ramp into repeated lethal combats.
Generates a second combat phase every time Bumi connects, doubling your damage output
Earthbend builds a wide, resilient land-creature army that dodges most board wipes labeled at 'creatures' less but survives via counters
Untapping all lands fuels big mana for spells between or during combats
Trample on a counter-laden Bumi gets through chump blockers
Animating lands exposes your mana base to creature removal and board wipes, which can two-for-one your lands and your engine
Heavily reliant on Bumi connecting; if he's removed or chump-blocked the extra combat is wasted
Vulnerable to fliers and evasive blockers since land creatures are usually grounded
Mass land destruction or Armageddon effects are catastrophic for the deck