These are the clips that hold the front of the undermount slides. Actually, the technical name for them is 'nipples' (teehee) - crazy europeans. At first glance they look pretty chintzy but they actually do their function very well.
The back of the slide has a little tab (that, incidentally, seems much more nipple-like) that needs a corresponding hole in the back of the drawer to engage. Originally I made a much fancier jig to locate and drill the hole accurately but it didn't work as well as this simpler version. Sometimes you just have to nail some scraps together.
Tab A into hole B.
One of the nice things about under-mount slides is their adjustability. This little blue wheel moves that wedge-thingy in and out, which moves the drawer up and down on the slide. So if the drawer front is a bit out of alignment you have about 1/16" of up & down adjustment on each side - allowing some fine tuning of the final fit.
...And it snaps in. Easy-peazy.

It's kinda nice to not have to see the slides.
