This is Maclan DRK Super Capacitor.

Designed for 2S No Prep Drag Race
Best performance with DRK ESC
9400uF total capacitance
Retains better battery voltage for higher RPM
Weight: 25g with wires

Note: This is an optional part that requires highly skilled soldering techniques to attach to the ESC without causing irreparable damage. Please have a professional technician perform the installation. If the ESC is damaged due to this service, your factory warranty can be affected and or voided.