I'm guessing they mean ranking to match for residency.
First and foremost, I think it would depend on the program. Not every place is going to interview, discuss, and rank candidates in the same manner or the same timeline.
If your PI speaks to the PD after everyone has been interviewed, discussed, and ranked; it may not make much of a difference. If they speak to the PD before all that happens, then the PD could bring that up in a ranking discussion (if said program has such a thing) and it could maybe make a difference.
Based on my experience, if one interviews and is already in the middle or bottom, something like this, even if timed right, isn't going to skyrocket someone to the top. It can however make a difference when talking about where to rank people in that general area of people (Hey, their PI contacted me and said they were great...we can move them up a spot or two above these others who we ranked around the same place. You know, these few in the middle who we're having trouble figuring out which order to put them in).
TL;DR: Worth doing, but don't expect miracles.
