Effect of back pressure on the final product

Hi, I have been told that if we set the back pressure high enough in injection molding machine to mix the recycled material (PE) too good, the final product become more flexible than those times the back pressure is low (the material in this case is brittle). I wonder to know is it true effect on the material, if so why?

Polyethylene??? What are you molding (part?)?

All bets are off with regrind materials, unless you know the generation and are controlling it.
Record melt temperatures at both processes, the higher back pressure is probably giving you better flow (flexible).