The frenulum and the associated tissue delta on the underside of the penis below the corona has been described in sexuality textbooks as “very reactive” and “particularly responsive to touch that is light and soft.” The “underside of the shaft of the penis, meaning the body below the corona” is a “source of distinct pleasure.” Crooks and Baur observe that two extremely sensitive specific locations that many men find particularly responsive to stimulation are the corona, and the frenulum.
Repeated stimulation of this structure will cause orgasm and ejaculation in some men. 
Frenulum of prepuce of penis (Wikipedia)
“Citation needed”; they should come and look at this video … ;-))
Some men say the only downside of circumcision of the foreskin is how much less sensitive the frenulum’ of a circumcised penis is, versus un-circumcised.