Today, after quite an absence I decided to pick up the Hand Drill again. Since it has been a while (a couple of months), my hands and muscles weren't very conditioned. I decided to use thumb loops. Thumb loops are basically an aid to the hand drill to prevent your hands from moving down the spindle. I use very simple thumb loops by tying a loop in each end of a piece of cordage (this is Jute) and tying the cordage around the top of the spindle. You do want each loop to hang at least 6 inches , otherwise I find it restricts movement of your hands.
You place each thumb through a loop and then rotate the spindle between your hands, as per usual for hand drilling, applying downward pressure as you spin. The thumb loops keep your hands in the same position.
It can take a bit of trial and error to get the knack of it and there have been times when I've found it easier without! It is a much under-used aid within the bushcraft community, I think a lot of people frown on it as cheating but for me it is just another aid, and you still need to learn and understand the principles of the hand drill to succeed with it. I do prefer to be able to Hand Drill without Thumb Loops but at times it can be a useful tool in the box, and can aid learning and aid in conditioning your hands\muscles (as you can usually go for longer as thumb loops are not as tiring as moving your hands up and down the spindle.)