When you have a big poop, and it lands sideways in the toilet, so it suspends the water like a bridge, spanning from one side of the bowl to the other. Upon flushing, the water will just sink beneath it, going down the pipe, and fill beneath the newly constructed "bridge". (The only way to flush a "suspension bridge" is to chop it up with a plunger).
"Man, I haven't taken a dump in 4 days. I totally just took one and made a giant suspension bridge."