The round stock has been inserted through the holes in the support. Push the drill bit and the round stock into one another. Because my motor turns at 3,000 rpm, I drill in very short bursts to keep the bit from overheating. Some radial arm saws have a 1/2 x 20 spindle that runs at a greatly reduced speed and is helpful for drilling operations.
The jig for this operation is specific to the size of the round stock used. It would be difficult to remove the jig and attach it again later. Too many things would have moved and the holes in the uprights would no longer be concentric with the drill bit. But, it is very useful when you have a specific need for a concentric hole in a longer piece of round stock.