My personal interpretation, and I emphasise this is MY interpretation, of the will to power is the defining characteristic of the Ubermensch, the force of the functioning mind to discover and then pursue the true meaning of one own life, to create and to contribute something new to the collective whole, to assert change.
This contrasts with the mind of the herd human whose own will does not drive them to power, but to serve, to "borrow" the will of the Ubermensch and to act upon it and constuct the new to the pattern provided.
The opposite of the will to power, the will to depower is the characteristic of the Letzter Mensch, the determination to half progress, to be reactionary, to regress the collective whole.