When having oral sex, the passive partner generally inserts his penis into the mouth of the active partner who then again stimulates the penis via mouth and tongue for sexual excitement.
However, during a mouthfuck the usually active partner changes roles with the passive one who normally receives the blowjob.
The result is that the partner having the penis in his/her mouth remains passive (unlike during normal oral sex) while the partner inserting his penis actively rubs it in his partner's mouth, often very quickly for maximum sexual pleasure.
For even more excitement, the active partner can also hold the head of the passive partner and move it back and forth as if he would hold the waist of a female during intercourse in order to guarantee the continuous act.
Disadvantages of this variant of intercourse mainly concern the passive partner who can be seriously injured when both partners are not cautious. That is because the head and the mouth simply are not robust enough for this kind of action exposing the passive partner to dangers such as neck injuries or even the risk of suffocating. Especially if the active partner is strong, performs the act at a fast pace and additionally secures himself by holding the passive partner's head, the chances of an accident are very high.
The active partner actually being required to be active is also a big drawback of this sex variant as males typically enjoy a blowjob when relaxing and thus while being passive.
An important point is that a mouthfuck can be usually only performed actively by males. Strap-on mouthfucks are not very recommended as the chance of injuring the partner increases due to the (usually hard) plastic dildo.
"Did you know that Gary enjoys mouthfucks?"
"Jeez, what a gay ass"
"No no, with his girlfriend!"
"Oh... damn I wish I also had one to mouthfuck." :(
"Honey, what about trying something new this evening?"
"Oh God, I knew this moment would come sometime... what today? If it's anal, count me out!" :P
"err...yeah it was anal." -_-
"Well how about a mouthfuck? Isn't it as good as a... you know, intercourse in the ass, hmm?" :)
"How 'bout you mouthfuck yourself!?! I wanna buttfuck you right now!" :/