Of all the types of industrial sewing machine mechanisms in common use, one of the best systems for feeding multiple layers of dense, thick, or slippery material, including leather, is the "walking foot" mechanism. Walking foot machines are a staple of the canvas, denim, leather, upholstery and vinyl sewing trades.
There are a few different variations of the walking foot feed system, but the most common one is the triple, a.k.a. compound feed system. Compound feed sewing machines have two alternating presser feet which move the material by synchronizing the motion of the needlebar, the inside presser foot and the bottom feed dog. While the material is being moved (forward or reverse), the outside foot is lifted off the top of the work. This allows the needle, inner foot and feed dog to easily transport all of the layers together, until the preset stitch length is reached.
When the feed mechanism reaches the preset stitch length, the outside foot lowers under spring pressure to secure the work. As that happens, the needle begins to ascend; forming the thread loop that goes around the bobbin, then fully withdrawing from the material. The outside foot remains firmly down on the material as the needlebar and inside presser foot bar move into position for the next stitch.
Because of this triple feed system, dense, heavy, slippery or multiple layered material will not drag or go out of alignment. Stitch length usually remains constant, from stitch to stitch. Finally, the lowered outside presser foot keeps the work from moving until you want it to (by raising the feet via the hand lifter or knee lever).
Walking foot machines are able to climb up over new layers of material, and climb down to lower levels, without hesitation or the need to manually raise the feet. Once properly adjusted for sufficient lift, the alternating foot action ensures that the inner or outer foot will grab the top of the new layer and the entire set then walks over it.