The Beak Buster is an attack learned from Bottles in Mumbo's Mountain. When 
is pressed any time in the air (making the move commonly used with 
+), Banjo will flip upside-down and Kazooie will pound the ground with her beak. It is arguably one of the most useful moves in Banjo-Kazooie since it is needed to pound floor switches and can also be used to break fragile objects such as wooded huts. The Beak Buster can also deal heavy damage to enemies, but getting above taller enemies to use the move can be tricky.
The move is kept intact in Banjo-Tooie, but its uses are few because most switches are weight-activated and the Bill Drill is used more often.
A tactic adapted from the "Buns of Steel" technique from Super Mario 64 can reduce the amount of damage taken from long falls. If the Beak Buster is used at the bottom of a fall, Banjo instantly loses all his downward momentum, essentially starting the fall over again from a lower point. This is more useful for shorter falls than extremely long ones, since Banjo cannot use the Beak Buster if he has been falling for too long, and using it too soon can lead to taking more damage than from the fall alone.
When Banjo hits the ground after a Beak Buster, he bounces upwards a bit. This "rebound" makes Banjo completely invincible, including from fall damage. Experts use this trick to fall long distances safely or avoid certain other hazards.