I have never held the skin at the base back when doing manual stretches. But if you're worried to increase your foreskin, you can hold back the skin at the base and then stretching with the other hand.
Depending on your circumstances, and how much length you plan to get.
When stretching
BTC I believe it's not possible to hold back the skin at the base, but I may be wrong.
As I said I have never held back the skin at the base.
Always pull the foreskin back a little bit when stretching.
Feels it's time for me to create some more video instructions.