What it is: Single Leg Slideboard Leg Curls
Why it is sick: Many muscles work across multiple joints. The hamstring is one of the big ones. It works as both a hip extensor and a knee flexor. Thus, it should be trained in both actions. Training the hamstrings with deadlifts, squats and cleans is fantastic, but the accessory work is just as important. Glute Ham Raises (GHR) and slideboard curls are a great way to achieve the other actions of the hamstring. And it does it in a much more dynamic way then your fancy Nautilus Hamstring Curl machine <vomit>. Plus, the hamstring is one of the most important muscles in explosive athleticism that many of us are looking for.
If one leg is too difficult, using both legs is a great way to start off. Another way to make the single leg version more difficult would be to hold your non-working leg against your abdomen, squeezing a tennis ball in your hip (a la the Gray Cook Hip lift). That version will really test your hip mobility.
GO DO IT!!