If you want to cook just one piece of toast (or a Pop-Tart et al.)? My toaster just like most of yours has a little icon showing where the single piece should go. How does the toaster “know” that that side is the one you’re going to use, much less whether you’ll actually use both sides, which supposedly cooks both equally.
My understanding is the sensor that tells it when the toast is done is on that side. If you put a single item in the wrong side and leave the sensor side empty, you’ll either under- or over-toast your bread.
If you’ve filled both slots, the machine gauges the doneness of both slices by the doneness of the slice in the sensor slot.
Depends on the toaster. Some have grids on both sides, others have “bagel” setting which only activates the grids on one side.