We used those in Germany, but the ones we had were mounted over a table type deal.
It has been a while, but if I remember right, you adjusted the table a bit up or down for the "basic" size wood you were doing.
You set the wood down flat on the table, then jammed it forward onto the spinning screw.
I used the stuffing out of that thing, and that other type saw in another thread so that we had firewood to heat the house (yes, that was the only heat)
I may have been too stupid to realize the dangers, but it was as safe as many of the things I was doing at the time.