He was actually referring to a DRDOS attack - check it out
here. In short, packet reflection is done by modifying the headers of packets (VERY large amounts) to make it look like it was coming from a different server. You then send that modified SYN packet to the destination, and when it returns with a SYN-ACK, it will respond to the server that the packet instructed it to (hence the modification).
IE: If I wanted to use my computer to bounce a packet from say, linuxquestions.org to your computer, I would modify the packets leaving my computer, to point to your IP address. LQ would recieve said packets, and, thinking it came from your computer, return a SYN-ACK packet to your computer.