The bagel button makes only one side of the slot get hot (or at least makes one side hotter than the other). Your toaster probably has a bagel icon on the top showing you which way to put the bagel in (cut side towards the center in every toaster I have seen).
Source: I'm not married to toaster moguls but I do peer into operating toasters to see which wires turn red. Feel free to replicate my research and post your observations.