
First let me preface this by saying I'm not certainly not the first to come up with this technique. Most of this is application of things I've learned from Neil van Niekerk's site, specifically here. He's got some of the best resources for on-camera flash.
When photographing events, I commonly run into the same problem, a group of people are talking and I'm stuck behind them. Taking a photo with a typically white card give me problems; there is still direct flash or high intensity bounce (off of the white card) going straight forward. Because light follows the inverse square law the light hitting the closest person back of the head is that much more powerful than my subject's face. Here take a look at this example.

For this photo I have my flash pointing straight up with a white card between me and the flash head. If I would have tilted the flash forward, the effect would have been more pronounced as there would have been even more direct path from the flash tube to the back of the head. there is still bounce getting to my subject, but it's lost with the direct light.

The trick is to flag off the direct light, so only bounce is available.

Here's another look at the result:

Other photographers may laugh at you because everything they've learned says "white card on the back, 45 degree bounce" but the light quality speaks for itself. Thanks Niel for all the pointers!