here is why you can kill yourself with the knife-and-toaster trick.
the AC power line into which the toaster is plugged consists of a hot line (black wire) that has 120VAC on it and a ground return line (white wire) which is at zero VAC or very close to it.
The toaster has a switch inside which feeds power to the bare resistance wire inside the slots when the lever is pressed down and turns it off when the toast is done. this looks like:
black wire (120VAC)------switch-----resistance wire-----white wire (0VAC)
On older toasters and older outlets, the power plug can be inserted into the power outlet in one of two ways: one where the hot wire leads to the switch (as shown above) and one where the hot wire is connected to the resistance wire. In this configuration, the resistance wire is at 120VAC even when the toaster is off.
In this case, if you touch the resistance wire with your knife your hand is connected to 120VAC which wants very badly to flow to ground potential. If the floor is concrete and just a tiny bit damp, your body completes the circuit and you get shocked badly. If the floor is dry linoleum, you may get a surprising tingle.
Note that this can happen EVEN WITH THE TOASTER OFF.
On modern toasters and modern electrical outlets, the plug and outlet are designed so you cannot stick the plug in "backwards": the hot side will always be connected to the switch, so that if the switch is off, the bare wire inside the toaster is at 0VAC and touching it with a knife will not shock you.
Now we treat the case where the toaster is ON and you jam a knife down the slot WHILE IT IS TOASTING, and touch the resistance wire inside. The end of the resistance wire that is closest to the switch is at 120VAC and the end that is closest to the white return wire is at ~0VAC. In the middle of the resistance wire length, the voltage is at ~60VAC.
If your knife touches the wire at a point along its length where the voltage present there just happens to be sufficient to establish current flow through your body, you will get shocked (see above).
If you are lucky enough to have a toaster with a 3-prong plug, then all the metal parts inside the toaster that are NOT touching the resistance wire are tied to a good electrical ground. this includes the thin wire guides that prevent the toast from touching the hot wires. in this case, if you jam a knife down the slot the chances are your knife will touch a guide wire BEFORE it makes contact with the hot wire and if the knife then touches the hot wire, the current will be shorted to ground via a better path than that provided by your body, and you will not get shocked.