TheSkySearchers

He is getting more mag by decreasing the aperture and increasing the focal ratio. Left Click the image.

He is describing a side effect as if it were a main effect.
By masking the aperture he decreases the effects of external seeing. No wonder he sees planets and very close double stars better.
As a matter of fact he decreases the contrast (a small, hardly noticable percentage) by having a relatively bigger central obstruction, but the gain in details as a result of the better seeing wins apparently.

Observing more details in an amateur 4 inch f/15 achromat compared to a 4 inch f/5 achromat is quite obvious though.