The porn video tag "sissy-riding-dick" refers to a specific sexual activity involving one person, usually male or gender non-conforming, riding on another person's erect penis. In this case, the term "sissy" is often used as a playful or submissive role within BDSM or kink play.
In the context of pornography, this tag describes a scene where one participant assumes a position such as cowgirl or reverse cowgirl and moves up and down on their partner's penis. This can be considered as a dominant act by the "sissy" character, who is in control of the pace and depth of the penetration.
This type of scene can cater to various fetishes and desires, including humiliation, role-play, and BDSM dynamics where one partner assumes a submissive or feminine role while the other takes on a dominant position.