When I think Chuck Rainey, I actually think of him tending to play on top a lot, depending on the records. That's just my association with him.
Maybe he is a hair behind, but it's a really subtle difference. You CAN play behind the beat while the drum machine is looped or whatever. I think better examples are any Bob Marley recordings, George Porter Jr with the Meters, Pino with D'Angelo. At least they're more obvious examples. Bootsy, Me'shell Ndegeocello, too.
Larry Graham is nice and behind here
http://www.youtube.com/watch?v=64OcpQC25V4
and Meshell's got a good one here. The bass doesn't come in til the end, but you can hear how behind the whole groove is.
http://www.youtube.com/watch?v=SqV9Z5DqUKM