The one I have has a centre pin that sticks down, a hollow circle around that, then as you say a couple of cutting edges on the outside.
Only used it a couple of times, but found the centre bit drilled right through, but the outer only drilled as far as the hollow bit would allow it, till it bottomed out on the weld.. I hope that makes sense.
I found I had to sort of rotate the drill at an angle to finish cutting all the way round the weld on some thicker panels, but it would not work on any panels where the spot weld went through 3 panels.
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