LAright Denny, here's the lowdown. The screwdriver part does indeed slide up into the square, and is spring loaded. The knob is only attached to the screwdriver part. On the square part, that's actually a 1/2 square drive, it's actually solid with the handle part. You use whatever size shallow socket that the engine would require, as it's made to be able to work with multiple engines. The item you see on the side is a ball type socket retainer, just like on a ratchet. I beleive the original intent was for use on Deisels, namely Cummins.
As far as an advantage? The knob on the top is easier to hold on to, particularly when using it on a running engine. That is the actual design intent on the tool, to be used with the engine running. When using it, the only part you hold onto is the knob, you can use your other hand to hold the feeler gauge, then once you get it set, you can lock it down with the hadle while holding it in place with the knob. It does work rather well, but doing it static, a stubby and a 5/8ths wrench does the job just as effectively.
