For those of you who have had the fortune of meeting Johnny or hearing him speak, he is still as charismatic and inspirational as ever. It was several years ago at the Tour de Cove that Johnny first tried a hand-cycle, and the idea for the Krankcycle was born. You can read about it here on Johnny's website.
Kranking requires full engagement of arm, shoulder, chest and back muscles. Stabilizers include all musculature in the torso, primarily the abdominals and low back and the rotator cuff muscles in the shoulder. In a standing position, the muscles of the lower body also engage for increased balance and stability.
There is a resistance knob to vary the force that you apply. You can also work each arm independently. This photo below is not very flattering (of course, one is always more critical of photos of oneself!), but you can tell I'm working hard with one arm. In doing so, you can really feel the work in the rotational muscles of the trunk.
As Johnny says, the Krankcycle competes with nothing, yet complements everything!