Sherwood modified it so that he could take out raised bumps or bows out of the middle of cleft wood. The centre or the domed part of the axe is going to hit the wood first and so it can be used very precisely and will not cut the outer edges of the wood. Mainly used along the grain. Other axes do work but with a rounded blade this axe will only cut on a small section or arc of the blade.
The bevel is huge and being on the inside it needs a lot of work to make it more acute. The obtuse bevel can be sharpened razor sharp but the problem is, as the tool slices through the wood, that this huge bevel will slow it down. A simple analogy is try pushing you fist through sand on the beech and then pushing your hand with your fingers tips first. The obtuse bevel will mean you will have to put a lot more effort, or energy, into the tool.
The sweep, that is the curve on the underside is steep and so will only take out small bites of wood, not what you want for chair seats where a shallow sweep is best.
The worst bit is that the edge is domed. WTF, who designed this tool, not someone who ever used an adze for chair bottoming. The shaving that can be taken out is small and you will be there for hours trying to hollow out a seat.
Now there well may be people who have found the perfect use of this tool, please let me know. For me and my uses this tool does not work and please do not waste your money on it.
Trevor, a student of mine, has just bought a second hand Gransfor Bruks two handed adze. I like it and would consider buying it for bowl work and chair bottoming. The sweep is less steep than the hand adze but still not quite flat enough for chair work. It works nicely and on a bowl it will waste the wood with little effort, whereas with the hand adze you will be still bashing away with an achey arm. I am still not sure about the GB hand adze, something not quite right about it. If you have one do grind the bevel further back, I have found this helps.