Don's plate stops the advance. To then get the initial and maximum where they should be, the distributor has to be rotatated. That shifts whole curve up. Alternatively the springs can be messed with and thats not easy for most folks. The secondary spring is the most critical and hardest to find for an advance that's been stopped early.
Welding up the inside of slots it cuts off the bottom of the timing curve. For most electronic distributors, that's it, all done. Curve is still correct when the initial is set like a pre-CAP engine (or higher initial if its hot cam).
