Sowell's A Conflict of Visions: Ideological Origins of Political Struggles then argues that the definition of power in the unconstrained vision, as evidenced by consitutional scholar Lawrence Tribe, is about results:
According to Tribe, if the government refuses to pay for abortions by indigent women, then it causes "coerced childbirth," acting in effect to conscript women (at least poor women) as "involuntary incubators," thereby "denying women power over both their bodies and their futures." This is consistent with the general logic of defining power in terms of the ability to change someone else's behavior, though inconsistent with the definition of power as the reduction of [others'] pre-existing options. [pg. 180]
Note that Tribe is concered with the "ability to change". This is not saying that someone else's behavior does change, just that you can influence the choice. To Tribe, this is power. In the constrained vision, this is not power because people still have options; and it is only in reducing options that one is powerful.