Morphing Ball bouncing (referred in sentences below as simply "bouncing") describes the effect of Samus bouncing due to landing on the ground while morphed.
When Samus hits the ground while morphed, Samus will bounce once before becoming fully grounded, where her speed is then reduced to that of Morphing Ball's roll.
Pressing down on the directional pad twice in midair will cause Samus to morph in midair. To retain Samus's horizontal velocity, you will need to hold the jump button while pressing down, and hold forward after morphing.
Bouncing can be useful in a situation where numerous small platforms would otherwise force a slower route through the area. Viable spots to bounce include the first pillar in the Cathedral or the robot in the Basement.
Performing a bounce while entering water (or lava/acid) without Gravity Suit will cause the speed before hitting the water to be maintained into the water until the end of the bounce. This is known as "waterball."
The reason this works is because while morphed, you maintain all of your speed when entering the water. You can even keep blue suit from Speed Booster if you jump into the water from the ground, even if you do not have Gravity Suit equipped.
You can also use the mockball technique underwater if you build up speed on ground and then jump into the water.
Since gravity is different underwater, you will bounce much higher and can keep your speed longer, even though the water slows you down due to friction.