In drumming, a gravity roll is a percussion technique. It is performed by resting the shaft of the drumstick on the rim of the snare drum, while holding the butt. The hand is moved up and down, causing the head of the stick to strike the drum's head once per stroke and the shaft of the drumstick will strike the rim of the drum on the way down (similar to a seesaw). The spot on the drumstick varies, as this technique requires considerable forearm precision. It helps to press down fairly hard and also following a slow to fast to slow pattern in order to gain more control.
The gravity roll can also be extended into a gravity blast, where bass drum and cymbals are added to make a blast beat. Currently there are at least three instructional videos on the market that cover the unique method. They are "The One Handed Drum Roll Training Pack" by Jared Falk, "Extreme Metal Drumming 101" by Flo Mounier of Cryptopsy, and a "Various Drum Techniques" video by Johhny Rabb.
When implemented in death metal the gravity roll is applied as 16th notes to the snare - typically with the right hand - and the left hand plays 8th notes on the hi-hat. The feet usually divide the beat as 16th notes, essentially matching the strokes of the right hand. In other words, the right hand is playing the same speed as the feet with the left hand keeping time with 8th notes on the hi-hats. However many variations exist, for example playing eighth notes with the feet instead of 16th notes or 32nd note kicks. Whatever the variation used, it is typically referred to as a gravity blast, a term which references the blast beat.