The Spring-Loaded Tray
The photo below shows you the view down one of the slots of a typical toaster. Two mica/nichrome sheets line either side of the slot. A metal holder rides up and down in the slot to raise and lower the bread.
Many toasters include a pair of grates on either side of the slot. The grates press against the bread and center it. Two metal springs get pushed when the holder nears the bottom of the slot, and they pull the grates inward.
The holders in each slot are connected to the handle that you depress to lower the bread into the toaster, as shown below: