I mentioned earlier that this was a double drive model. It will actually operate as single or double drive. Here’s what that means.
You can see that there is a second groove on the flyer pulley. This allows a different drive ratio to be used. All this movement of the belt changes the tension but there’s a cunning knob and hinge arrangement to retension it.
Specifically this is “bobbin lead” or Irish Tension single drive. Alternatively, you can single drive the flyer pulley and brake the bobbin. That would be “flyer lead” or Scotch Tension single drive.