On the 111W117 there is a clamp atop the extended lifting presser foot bar as dikman has indicated. Attached to that clamp would have been an extension that goes down in front of the machine and rests at an angle on a sliding collar. The collar is on the on the upper thread tension post where the adjusting thumb nut would normally be. As the thickness of the material increases, the angled extension rises and begins to relieve pressure on the beehive tension spring. Upper thread tension is reduced.
The lifting presser bar has a flat spring as most 111W machines do, though of a different part number than other flat springs as it has attachment points. There is an auxiliary bowed spring attached atop the main spring and this auxiliary spring is extremely light. The adjusting thumb nut is shorter than in other 111W versions to allow for the bowed spring. All or almost all foot pressure comes from this extra light auxiliary spring.
In its intended use for stitching sleeves in coats, the mechanism functions to allow maximum fullness in the thick padded portions, so that the stitches do not compress the padding. The resultant stitch in thick padded portions would be somewhat close to a basting stitch as the bobbin thread would only be drawn slightly into the material.