Maybe someone more in tune with the physics of the game could provide an explanation as to why the spin would help, but I haven't seen anything in my experience that would lead me to believe that the spin itself would cause the OB to track down the rail more consistently.
The best tip I ever got for cutting frozen balls down the rail was to aim to hit the rail very, very slightly before the ball. The difference in aim point may only be a millimeter, and it is so close that you really don't know if you hit the rail or the ball first, but my percentages on these shots went way up when I started doing it. Based on that, I would submit as a possibility that it is not the inside spin itself, but rather the squirt that is assisting you with this shot. If you are catching the OB slightly thinner due to the squirt, that could result in the same positive effect I see by aiming slightly thinner without spin.
Aaron