Yeah, granted some actual motion would have been helpful LOL....
I've actually heard it used in ghetto vernacular many times, mostly when describing someone's arse, ie, "Now that's what I call a backfield in motion brutha" etc, etc......
It's cropped up in a few songs since mind, principally in E.U.'s 1989 smash, conveniently named, "Da Butt" - a song which is pretty much devoted to the wonders of the female gluteus maximus .....
"Notion, put your backfield in motion,
Hey doin' the butt.
Hey sexy, sexy
Ain't nothing wrong,
if you want to do the butt all night long"
And naturally, rap music clasped the 'backfield in motion' terminology to it's bosom, check out Grandmaster Flash's "Them Jeans"......
"Walkin' through my neighborhood
Her backfield's in motion
Clingin' to body smooth as lotion
Struttin' that stuff so firm and lean"
Pretty standard stuff in the hoods I hang in.........
Ian D