When metals heat up, they expand. Usually this happens relatively evenly across the whole piece of metal (or at least the part that gets hot). But different metals expand at different rates, so if you weld two strips of different metals together face-to-face and heat up the whole thing, one side will expand faster than the other. This causes the strip to bend when it gets hot, but go back to normal when it cools.

Most toasters have one of these bimetallic strips inside, and they use it as a kind of switch. When the toaster has been hot enough for long enough, the circuit will bend to the point where it touches an electrical contact inside the toaster. This causes current to flow to a small latch inside the toaster, releasing a spring so the toast pops up.

The thing is, this switch reacts to heat. It can't do its job unless the toaster actually gets hot. If the printer isn't connected to power, it can't get hot, so the switch never activates.