Problem Solver's is for chucking in a drill/driver. The shank is 3/8". It's not supposed to be used freehand, but you could if you really wanted to.
The center is hollow, for inserting your own cut spoke, which you can set at whatever depth you want to push the nipple off. When you are building a bunch of the same wheel, you can get it set just-so, so that when you are done driving them on all the way around, you can head straight to tensioning the wheel.
It expedites building wheels...that's all. If you know your target spoke length, and where the spoke ends will wind up, you can set the Holy Driver to push off 2-3mm before that, and it'll place you in the same place. Takes me 2 minutes, give or take, to get the 'pre-tension' phase done.
I modified two regular flat scredriver bits used it with a low torque B&D driver, one has the center protruding (same as the Park Tool in the above picture) and the other has a slot in the center (same as the Problem Solver picture above), for when the spoke starts interfering with the slot in the back of the nipple. I built the last set of wheel with DT Swiss squorx nipples so I had to use the DT Swiss specific tool.