http://zzz.i2p/posts/13188
Refering to the picture in the link in #5 above: If Alice sends, say, a 1KB garlic message to Bob, and gets clove 2 (the acknowledgement) back from Bob, then Alice has proof that routers C, D, and E relayed 1KB, and that routers J and K relayed a small ack (maybe 100 bytes after being wrapped up). So that's Alice having proof that C, D, E, J, and K did something (relaying) which is directly beneficial to Alice.