Meaning 1.
A Homosexual man that rides the knob/head of the penis to minimize penetration/painfulness during anal intercourse.
Meaning 2.
A anal retentive (tight assed) person that won't do a friend a small favour. The payout factor is that usually during sex the entire penis (shaft and knob) penetrates or is riden. This meaning suggests that via being a knob jockey and not a cock jockey that they are not giving the other party the satisfaction of riding the entire shaft rather than just the knob. Somewhat of a bastard.
A person(male) who likes to be ridden like a horse by another man. The ritual includes the 'rider' inserting his penis upthe other man (horses) bottom, thus giving an enjoyable ride.