I can now say I have successfully used the mouth drill ! But I won't be doing it again!
The mouth drill is of Inuit ancestry, where the mouth holds the bearing block and you apply the downward pressure with your head basically! I used it as a strap drill and used a thinner spindle. I needed lots of wraps of the cordage around the spindle to get the tension. It did work very well but it wasn't very pleasant!
I’ve seen photos of Inuits using a bow drill and they hold the set on their lap whilst sitting (that has been more for drilling than fire making.) I carved a bearing block so I could hold it in my mouth but I have also seen Inuit bearing blocks which fit onto the chin rather than the mouth.
I think the mouth drill was used more for drilling (not fire making) - and they would use a bow so it would free up a had to do other things!