I've called it twice - what I'm looking for is something like what Reggie Bush did to Leinart on the last play of the USC-ND game (the most blatant, public example of this foul I've ever seen, including braggadocio by the perpetrator after the fact). Hands on the back of the runner, obviously pushing.
Leaning into the pile, whether contacting the ball carrier or not, is not likely to get the foul.