I used mine for the first time in months the other day when I was concerned that she might have flipped into a breech position. I figured if I could find that really loud heartbeat, where I know I'm right on her chest, and it was below my belly button, then she was most likely still head down. But if I found it it the upper half of my belly, then she might have turned. Now that was just my logic, it's very possible that there is no correlation there whatsoever. So take that with a grain of salt. But in my case, I found it on the bottom half of my bump and she's still head down.
ETA: I'm curious now. Any breech Mama's out there with a doppler care to test this out?