Its about change, i.e. position change in porn movies.
Difficult to catch for those shooting their own screencaps. And non existant with most professional pic providers.
Because it shows kind of a bts, the makeing of the illusion, and its troubles, too.
First example is Czech cutie Julie Silver trying to hop on Steve Holmes pole for some anal workout wearing about 10" high heels. Julie takes her hand to assecurate herself at the back of the sofa and manages with style. 
