Its primary goal is sexual arousal in its audience. Mutual masturbation, mutual manual stimulation of the genitals between partners, can be a substitute for sexual penetration. Foot fetish pornography features heavy emphasis on the feet of the participants, either in combination with kissing and genital sex or in the context of acts such as trampling or crush fetishism. In POV porn, the shooting style is generally similar to gonzo pornography, with the person receiving sexual gratification holding the camera him or herself—aiming it down at the actress or actor who is performing the sex act. The act may be carried out face to face, or head to tail, with the woman recommended to be on top if she has smaller breasts. They may regard it as an enhancing element of their sex lives or as their primary form of sexual activity. Commenting on why there are not as many female macrophiles, psychologist Helen Friedman theorized that because women in most societies already view men as dominant and powerful, there no need for them to fantasize about it. Hardcore pornography is pornography that depicts penetration or extreme fetish acts, or both. For example, in Australia it is rated Refused Classification . Some pornographic hentai porn films use an artificial semen substitute to simulate momporn or enhance creampie shots. It is also known colloquially as gushing or squirting, although these are considered to be different phenomena in some research publications. For some people, non-penetrative sex is the primary sexual activity of choice above all others. The industry generally refers to such films as adult films, which is part of adult entertainment. Hamedori is a genre of Japanese pornography in which a male adult video actor or director serves as the camera operator. Interactive pornography, available in CD-ROM and DVD format, allows the viewer to select the particular acts performed at any one time, sometimes with the camera providing a first person view. Bukkake pornography involves a hot gay sex performer, usually female, onto whom several men or women ejaculate, usually on the face.