But what I don't get is that at least one set of the prongs must be
hot. How would they connect anything to it without getting zapped
?

Actually, both legs are hot. Each leg is 110v, so together you get 220v
service. The third leg goes straigt to the circuit panel and that is
the return, so for 110v, you are using one of the legs and the return.

Anyways, if your jumper is plastic sheilded you would be alright, or if
you wear ruber or leather gloves. Another very common senario is that
theives steel the glass meter from a vacant property. Then they simply
swap meters. The stolen meter would be used for two to three weeks, and
then swapped again until the meter reader comes by. This way all the
time that electricity is being used while the stolen meter is inplace,
is not being charged to the occupyer. If one does that for 15 to 22
days a month, it results in in 50% to 75% of the electrical usage not
appearing on his bill.