The only donut I've ever heard of in reloading is the one made by sizing down a cartridge to some wild cat. Where I read about it the most was in creating Ackley Imp case's. What happen's it the case is sized down but falls a bit short of the junction of the neck and shoulder. Do that and look at the case and there is a little hump left, donut. As I understand it it is one way of first firing the new case. The donut hold's the case in position for fire forming. The other way I've read about is seating the bullet out into the lands a bit to hold the case in place to fire form.
Don't know if it really works or not, would thing the donut might though. I've sized down a lot of 308 case's to 243 without worrying about putting a donut on them. Also do 30-06 case's and 270 case's to 6.5x06 without a donut, no problem. Might add that in all my rifle's after the first firing, I partical resize for that rifle's chamber. I have 2 243's and bot has their own set of dies set for their own chamber's. One chamber is enough larger that case's for the other rifle won't chamber in it! Might be a good idea to revisit the idea of a donut when sizing down 06 case's to 6.5, maybe!