Wrist straps are practically useless for good zapping and are only a convienence.
It is best to zap with copper handles and footpads at the same time for a 4 point contact system that provides 3 points positive and 1 point negative.
I personally would never us a zapper that plugs into the wall as I have heard several complaints.
As for needing more than 9 volts, this is not actually true. 8 volts works ok but may take a little longer. Too high of a voltage can cause neuropathy.