This the track proxy has some "double double buffering" going on. First we keep two mesh frames. The one just before the current time and the one just after the current time. This is the full mesh and we interpolate between it to derive the actual mesh for the exact time we're at. Secondly we have two position buffers. The one for the current rendered frame and the one from the previous rendered frame (this is not the same as the mesh frame, the mesh may be at say 10 fps then get interpolated to 60 fps rendered frames)