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....
This is a "violent" kind of blowjob.
you hold the female or male (shemale if wanted) head and fuck theem in the mouth!