Radical sinks draw energies from unequal social relations without changing those relations. They give rise to extrinsic motivations for radical action. Radical praxis draws energies from unequal social relations and uses it transformatively. It involves motivations intrinsic to radical projects.
We can define radicalism not by its conspicuous oppositionalism or militancy, but by its effort to intentionally transform social systems. The radical Left is a loose constellation of attempts to achieve maximal human freedom and equality through intentional social-systemic transformation. Any process which draws energy from that trajectory without returning it can be considered in terms analogous to a thermodynamic sink.