Also see
Various forms of draging
Isolating Muscle Groups
Ankle dragging attachment. This is a simple setup made with readily available materials, a leash and two dog collars. Make sure the strap that has the two ankle attachments on it is not fixed to the sled but passes freely through a ring on the sled.
Sideways ankle dragging works the adductors and abductors of the hip.
Ankle dragging forward, with the sled behind you, works your hip flexors to a great degree
Ankle dragging backwards, with the sled in front of you, works the hip extensors, the glutes and hamstrings, to a great degree.