Leave it to Martin Rooney to show me a bunch of stuff I have never seen before. This one of one many Sledge exercises he showed me during a trip to Jersey.
The sledge is a great training devise because the weight is distributed further away from the body. This is something you have to perform to fully respect.
Sit on the floor with your knees slightly bent and lean back until you feel tension in your abdominal region. Grab the hammer with boths hands (as pictured) and dynamically twist from side to side.
Begin with only a couple sets of 10-15 reps because you will get very sore from this one.