"Power exchange" in BDSM or leathersex refers to the consensual interplay in control between participants in a 'scene'.

While nominally the 'top' (or dominant) is in control at all times, because the exchange of power is based on consent, that control actually exists only in a narrowly defined context ... and it has limits.

If the scene in question has safewords in place then the bottom (submissive) can stop the scene at any time. Even in a scene between players you aren't using safewords, the top is always reading the feedback / experience of the bottom, thus the bottom's state is always setting bounds on the tops actions.

Whether a scene is based on SM e.g. flogging or sex-magic work e.g. fisting or DS/role-play, power exchange is an exercise in balance. The top(s) may experience pleasure from alternately pushing his or her partner(s) to some edge and then letting off, allowing stamina to rebuild and re-assaulting.

The pace of this 'dance' can be hypnotic, the pace of SM play is (not surprisingly) like the ebb and flow of any good sex, building repeatedly closer to (or through multiple) orgasm(s). The rhythms, like those of music often echo the body's rhythms of breath and heartbeat.

This is the difference between 'domineering' actions in which basically there is (potential) energy on one side of the equation, it's static (and not very interesting) it may indeed be quite draining to put energy into maintaining one-sided balance of power.

This is the essence of leathersex, the power exchange between players is dynamic and can build new energy in the process.

"If there is no exchange of power then it's not leathersex"

-- Joseph Bean, Leathersex