The Align motor heatsink is a small gold anodised heatsink designed to fit their own range of motors, specifically the Align 400S 2800kv and the Align 400L 2500kv.
The heatsink quite literally is a push fit onto the motor can. Once on it is fairly easy to remove again or swivel to align it within the Trex sideframes.
The Align motors are know for getting quite hot (too hot to touch) without the heatsink, so it is a worthy addition if you wish to prolong the life of your motor.