The filament tends to slip out of the hobbed area of the extruder drive bolt on my machine. I wanted to make a new extruder block, either thingiverse.com/thing:13530 or thingiverse.com/thing:14317 but of course, because my extruder couldn't be left unattended for more than a minute or two, and this is a big part, I couldn't ever get one finished.

So this is just a little insert that sits inside the extruder chamber, and prevents the filament from being squeezed out. It's dead simple.