To my mind it's a technique of creating a kind of vacuum around the shaft. I have three rubber rings of varying diameters and I have applied them to the base of my penis whilst flaccid; then manual stimulation till the penis is fully erect. You can continue to force more blood into it by exercising your muscles.
Years ago, I recall rupturing the odd vessel causing slight bleeding; however, I let it heal and persisted. It is a matter of dogged persistence - not giving up, if you are sufficiently determined.
My girth, fully excited, is up to 6". I guess that's one reason why girls can't take their eyes and grinning at it on naturist beaches (sorry, moment of immodesty)!