"backlash" is the freedom of movement in a mechanism. In this case, you squeeze the capo on, and because nothing ever fits perfectly, when you let go, the capo relaxes its grip just a little bit. COmpare it to a handbrake on a car if you like (yes, I know the mechanisms are different).

I quite like the Kyser pro-am (the cheap one) which does need two hands, but you squeeze it on, and then tighten up the thumbscrew just enough to hold it there (which also takes out the backlash).