The K/S video artist begins by cataloguing all the scenes of
Kirk and Spock together, and then selects the ones that when
matched with the music will bring out what the fans call the
“slash premise,” that is, that the two men are in love and sexually
involved with each other. As I said before, the advice ranges
from the practical “Look at your video material without sound the
sound will confuse you”; “Use the show’s own cuts, fades, dissolves,
etc. since you can’t do them yourself’) to the aesthetic
("Watch MTV or VH1 and just see what they do”; “Don’t be too
literal, for example, having a line like ‘rain is pouring down’ and
showing someone in the shower”).

Constance Penley, talking about a 1990s fanvid workshop at Escapade, a California slash convention from  her book  “NASA/TREK Popular Science and Sex in America”  (1997)