With a riding heatsink application, the heatsink surface drops into an opening in the cage top and makes direct contact with the transceiver module or cable plug when inserted.  The clip assures good contact between the heatsink and the hot surface of the plug, drawing heat out to the heatsink pins or fins.  Air flow can then draw heat from the chassis.  Riding heatsink options are available for SFP, SFP+, XFP, and QSFP interfaces.