...If you use a jam chuck...you still have to use the tail stock correct? So I assume you just turn it as close as you can then remove from lathe and cut or sand down the nub thats left?
That's correct. You typically use the tailstock with a friction/jam chuck. Actually, some guys make a jam chuck that's perfectly sized to hold the bowl without the tailstock, but that's more work than I'm interested in doing.
Here's a series of pics showing how I often do it...
This is a box elder bowl. This shot shows the tenon:
For the friction chuck, I'm using my vacuum chuck, but it's not hooked up to the vacuum pump. I add a ring of wetsuit material just as padding, to keep from denting the soft box elder wood:
Using the tailstock with moderate pressure to keep things in place:
As I whittle down the tenon, I define the area that will become the recessed (concave) foot. This is also when I'll do my final sanding on the lower parts of the bowl. You can see in this pic I still have some sanding to do, but in the next pic the sanding is done:
Then I continue turning the nub smaller and smaller, until it's a cone. Typically, I'll turn the cone tip down to about 1/8" then make additional cuts and do some sanding to further refine the foot. With harder woods like maple or cherry, you can get the point of the cone down to about 1/16" or smaller before it lets go. In this photo, the wood gave way and the cone came loose sooner than I wanted, so I didn't get a chance to dress up the foot the way I wanted to:
This photo is from a different bowl, but it shows how small I typically make the point of the cone:
Once the cone lets go, any additional finishing is done off the lathe. Here, I used a chisel to trim the leftovers from the cone. In most cases, I don't use a chisel because I'm able to make the final cut cleanly while it's still spinning on the lathe.
Obligatory action shot:
Then a bit of power sanding to finish up:
And it's ready for signing and dating:
Does that help explain it?