I only grasp the limbs with 2" diameter max, except with dead wood which can be a little bigger. That's about what I can hold with the thumb touching the trigger finger, giving me the best grip. I found that's a good size for most trees to match the max load I can handle. Cupressus arizonica for example has a lower limit due to it's density and its heavy clusters of fruits.
The way I understand the gizmo is to avoid the twisting motion on the wrist when the limb fall from upright position. The hand has to take the load however, but the rope follow all the angle's changes without complaining, so the wrist isn't forced sideway.
Usually, when you grasp full hand a limb in front of you or above, the wrist is almost in line with the arm, the thumb oriented to the butt side and the pinky to the tip, so the limb is perpendicular to your arm / wrist, like any tool's handle. After the cut, the limb falls tip first, pulling down your hand.
With a light limb, no worry, you can keep the hand in front of you, the fore-arm just rotates to accommodate the new position, the limb staying perpendicular to your arm, like you hold a candle and the wrist is fine.
With a heavier limb, the arm can't hold it like that and is forced to unfold, becoming vertical, in line with the limb. Problem, the hand has to hold it perpendicular to keep this heavy weight (best grip). The poor wrist is in between and unable to match this angle, locked and over stressed. Because of that, the hand can't fully follow the limb. Finally, the limb try to open your hand by prying out the pinky and the ring finger, so only three fingers are able to maintain the grip. With a heavy limb, it's painful and close to a oopsy.
If you hold the limb with the thumb to the tip and the pinky to the butt, like it would be ok after the cut with the limb dandling under you, the hand is palm up and the fore-arm can't rotate any more to follow the fall. The wrist and the shoulder take the beating (hard).
I've tried to find a better hand position / movement for the heavy woods, but no way.
I use a lot the self rigging technique with a bunch of slings which I always carry on my saddle. That's handy, from 2" to about 6-8" diameter, but it's time consuming, regardless of the diameter.
The cut release on the handle would avoid half of that time. If the setting around the limb can be quickly made, the Jomo's gizmo could be interesting.
But Sean seems right too, the perpendicular handle has better holding capabilities.
