In summary, Ferguson says why bet/bluff a medium-weak hand on the river because if you’re going to get called by your opponent, it’s almost certain that he has you beaten. If his hand is weaker, he’s going to fold. Thus, the right play with the medium-weak hand on the river is to either check-and-fold or check-and-call.
Put another way, as Ferguson says, “As a general rule you should only bluff with hands that have no chance or almost no chance of winning in a showdown. How else are you going to win the pot with these hands except with a bluff?”
And while a medium-weak hand can be used potentially as a bluff, it’s nature as a hand that can possibly win but may not makes it more valuable as a checking hand than a bluffing hand.