These DHT derivs can bind to shbg and increase free test. (At the same time, they can reduce endogenous test depending on circumstances, which can reduce test and metabolites).
Anyhow, assuming you are taking exogenous test, main effect should be to elevate free test. Not sure how much though.
All effect available testosterone. Primo acts as an aromatase inhibitor to some extent (aromatase enzyme converts androgens into estrogens) it also decreases DHT & promotes LH production while decreasing SHGB. This results in an increase to natural production of test while simultaneously increasing free testosterone. The flip side is primo can still induce HPTA shutdown, which will result in the pituitary to release less luteinizing hormone (LH), which tells the testes to produce LESS testosterone.
So the effects towards free test can be positive or negative. It depends on many factors. If you already don't have very high available test then a compound like primo would be favorable over most choices. Others like Masteron/Winstrol can also be used with a dose as low as 5mg/day to lower SHBG to increase free test. I have seen guys take very low doses of winstrol late into a test only cycle (10-15mg/day) and get a kick start with progress once starting this.