No need to back. They do require a bit of time but more importantly some axle movement to fully lock/unlock. Usually I hit the switch while approaching allowing about 5-10 feet for the light to go solid. Then when unlocking I just hit the switch and go - sometimes it takes a few feet to unload enough to unlock - especially if axle bound from trying to turn. I would say that if you need to turn tight so dont have the few feet to spare - rock back and forth to unbind and it should unlock pretty quick - i have had to do this when going from both locked to rear only - the fronts do bind more (as they are steering).
J Wm Bishop EA, ASADE
The wagon should, of course, be as light as possible, but strength should not be sacrificed to lightness, for on any but the regularly traveled roads, the wagon will get many a hard knock...