If there is (and almost always there is) a gap between the bottom of the slot on distributor shaft and the depth of the slot hole in the distributor/oil pump drive gear the the gear can bounce or move up and down that amount. Now as the gear moves up and down it turns back and forth which advances and retards the timing the more gap the more it advances and retards.

Remember as you install the gear you turn gear down in it turns as it goes in and has to be turned opposite way to gear gear back up and out. this action if not controlled by the collar happens all the time if it's a bounce will be seen as timing bouncing all over if it is more smooth as mine was as load was applied timing would retard and go back as load was removed. found mine was retarding about 5 degrees under load so after installing the collar did not move and picked up 1.5 tenths in the 1/4 mile! :-)

I had seen these on Hughes engines and thought some day I'll spend a minute and get my head around how it can help. so late one night after going to bed started to run it through my mind. then realized the direction my dizzy spins and visualized installing the gear that it would want to turn and climb under load. Built a shaft stop collar the next day and problem solved e-mailed all my buddy's explaining what is happening and why an now we all have the shaft collars! Thanks Dave Hughes.