Back Set Nurtrients

[Daveweldz]

Been reading this forum on back setting sugar washes.
Maybe I missed it but dont seem to see were you need to add more yeast nutrients when rechaging the mix, just sugar and water. Is this because there is some dead yeast in the solids left in the bottom of the fermentor and those provide the nutrients?
I also read the limit to back setting is 6-7 times, what limits the number?
Also that the quaility improves thru the progresive back sets, why ?

Thanks
Dave