Mark noted that the pistol grip portion helps with rotational forces - the rod portion doesn't really contact the forearm as far as you can feel - it's the "saddle" at the elbow which provides vertical stability using your upper arm and shoulder to counter the downward forces of the wood against the tool.
There are certainly some considerations to this style of hollowing:
1) Your forearm and upper body needs to be nearly in line with the center axis of the turning, which means you are leaning sideways over the lathe bed. If you can hollow straight on with a short bed or a swiveling head lathe, that would be better for your back.
2) Your elbow height in relation to the tooltip near vertical center needs to be taken into account. If you need to lower your upper body to make it work, you really don't want to bend down for any length of time - again, think of your back.
3) Lastly, some turners, (Christian Burchard for example) have stopped using this style of hollowing because of the long-term stresses on the shoulder that's absorbing the forces. I haven't felt anything like that, but I don't hollow anywhere near the number of pieces that Christian does/did.