"Gravity hax" is a controversial technique that causes gravity air shots to hit grounded opponents. It is best performed by Burning Beasts, Little Raiders, Little Sprinters, Old-Timers and the Chick robo. Its main targets are jumpers and high-flying robos.
To use it, almost simultaneously press the gun button and the jump botton. Keep in mind that you will have to stay at mid-range. If you are successful, the air shot fires very low, and hits grounded robos with ease.
The way it works is simple: the robos mentioned above hold their guns lower than any other robo, so their air shots are only marginally higher than their ground shots.