I respectfully suggest that theories that masturbation style or brand or size of underpants caused a curvature to the penis are probably incorrect. Possibly correct, but probably not.
I say this for a two reasons:
1) most (all?) boys masturbate in ways that are similar, but only a few have curved penises. If a certain technique caused it, then there would be more men with a bent penis. There are not limitless ways to masturbate. You can grab it, or turn your hand over and grab it, and so on. No technique seems to pose a greater stress to the penis.
2) most boys wear underpants, and sometimes they are small - but, again, most boys do not get bent penises.
So why would that be the case?
First, biologic variation, also known as “congenital curvature” is one reason. You were born that way. If one looks carefully at the face and compares left versus right side, you will see small variations - same with the penis. Sometimes, it is more severe, and represents an abnormal development. Again, “abnormal” is usually defined by function - if one cannot have intercourse, then one has an “abnormality”.
The second reason for a bend is an injury and (usually) an abnormal healing from the injury. Most injuries are felt to be minor, but the response is not small, and excessive scar tissue forms. This then becomes like putting a piece of tape on a balloon before blowing it up - it leads to a bend as the scar does not stretch out as well. This process is called “Peyronie’s Disease”. I personally know (via the web) men who have developed this using jelqing techniques too vigorously (or, they used it properly and were unlucky enough that their penis responded to is as an injury), and from the VED. The VED case was associated with extremely high vacuum pressures by a poorly educated user and was written up in the medical literature.
So, having personally developed a curve as a result of an injury, and having spent years figuring out what helps and what hurts me (and usually others), I would recommend that anyone who gets a curve in the course of doing PE *stop doing it* (ie give it a rest) and get help from those who have Peyronie’s Disease. If one fails to do this, they run the very real risk of the bend getting worse (and permanent), and developing associated ED.
Just my two cents worth..
Tim