Flagging

Flagging

Flagging is when a female dog pulls her tail to the side, exposing her genitals. This behavior is often associated with the female being in heat, although she may also decide to do this out-of-season in order to solicit sex despite not being fertile at that time. She may flag any individual she is receptive toward, without regard to their species. This will most typically be other dogs or humans.

Aside from holding her tail to the side and putting herself on exhibition, she may also back up against her desired mate, often looking back over her shoulder as if to further encourage them.

References

http://beastiality.club/beastiality-club-extreme-animal-sex-content/the-encyclopedia-of-zoophilia/