First as to terminology. The way I learned for a jamb chuck was like Mike explained. You turn a recess or tenon to match the diameter of your item. The part “jammed” is usually about ˝”. If you make it a little too small you can use paper towels to enhance the fit. This method allows you to turn the bottom with no support from the tailstock. It is tricky in that you must have a proper fit and use light cuts. In videos by Richard Raffan, Ray Keys, and Dennis White they all had to use paper towel to adjust the fit at one time or another so don’t feel bad if it is not perfect the first time, you really have to sneak up on the fit.
There is also a friction chuck and this sounds like what you are describing. Some people do call this a jamb chuck.
Any scrap of wood is held in the jaws and does not match your item in diameter (it is turned round and true). The end is flat or slightly concave so that only the outer edge contacts the inside of your item. You place whatever you want over the end, paper towel, scrap of leather, foam…. Anything to keep the inside from getting marred. You will need tailstock support for this, using only enough pressure for the necessary friction. Turn the bottom leaving only a small nub where the tailstock meets. Remove from the lathe and chisel off and sand away the small nub.
You can use either method. With the jamb chuck you have to be very accurate with the fit since this is the only thing holding the item on the lathe. With the friction chuck you have lots of leeway but you should anticipate your need to center the item and leave the original nub (or some mark from the tailstock) at the bottom from the start of your project. If you have the original point it is quick to line up the center again.
For a bowl I leave about a 3/8” wide nub when roughing out and it stays there until the very end of the final turning.
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