From Team Yokomo:
A capacitor for racing that maximizes the output performance of the speed controller. When the load becomes high due to sudden throttle operation, the power from the battery temporarily drops, but this capacitor temporarily supplements the power to increase the acceleration.
Each cord coming out of the capacitor is attached to the battery plus / minus connection terminal of ESC, but since the capacitor itself is non-polar, it can be used by attaching it to either connection terminal.
The case is made of aluminum and has good heat dissipation, and has a thin shape that is easy to install on a machine.