Some "weed chopper" types will lock on for a longer period of time to "burn off" encroaching weeds - but they have been outlawed in a lot of places for 2 safety resons - 1- not releasing to let a vicim get away, and 2- staring grass fires.
Here in Canada I believe the only approved fence chargers are pulsed - I'll see if I can confirm one way or the other.