The clock choke in my opinion is something every BJJ practitioner should learn before considering him/herself advanced. It is a variation of the lapel choke from back mount on a turtled opponent.
There are many variations of how to grip and apply pressure, but generally the principles are the same for all of them.
1) The choke is always done from a side-ride on a turtled opponent
2) The arm that is closest to the neck is always the one choking
3) the other arm is used either to tighten the choke, or to keep balance (and stop sweeps)
4) You must walk around their body in a circular fashion to finish the choke (not necessarily clock-wise because you can do it from both sides)
Now an amazing variation of the move done by Andre Marola