@rpm: That equation is the expectation of a bet on the river ignoring what is currently in the pot. Its basically assuming that we've already won that, so the EV of an additional bet is just how often we're called by how much we can win.

Thinking in a real example...if we have the nuts and bet nothing, our EV is 0. If we have the nuts and bet 10 and villain calls 50% of the time, or we bet 5 and villain calls 70% of the time...the $10 bet would be a better choice as we'll expect to win 10*.5 = $5 with the 10 bet...and 5*.7=3.5 for the 5.