ePSXe... On the rumble setting, whats the difference between "Constant" and "Sine"

When I go to gamepad and try to set up the buttons, theres an option for rumble (which i think is vibration). Theres a big motor and small motor? What does that mean? And when I try to turn on rumble, whats the difference between sine and constant?

The psx has two motors, a big and a small. The big is constant vibration, the small can change its speed. If you have a controller like the psx set small to sine and big to constant. If you have a knock off controller that does not look like a psx it may only have one motor in it. IF so, set it to Big->constant.