On the reachback, and pullthrough your throwing shoulder should be level with the other side, or slightly below the off-shoulder. On the follow through, keep your shoulders level (If you're throwing a flat throw), otherwise if you lift your throwing shoulder you'll have a wrist roll over, causing those drivers to flip due to off-axis torque. on the follow through, try to have your thumb nail pointing straight up. This is a nuetral follow through, and won't produce off axis torque if done correctly.
Snap is a hard thing to describe and/or teach. What helped me, was visualising my wrist's movements in the throw. When working on snap, use a putter or a midrange and do a static throw with 50% power so you can really feel the snap. On the reachback your wrist will be nuetral (In line with your forearm), as you pull it through it will start to close (Hold your arm out infront of you with the top of your have facing up, now make your wrist go limp. This is a closed position) infront of your sternum, and will finish closing about a foot past your chest. in the next foot your wrist travels you want your wrist to open up with as much snap* as possible (Think of snapping a towel, the faster your wrist cocks and snaps out, the better towel snap you get. Same logic in disc golf).
The way I go about applying it, is make sure i have a grip that almost locks the disc in my hand, so i can have a freely moving wrist without having my disc wibble wobble about. With this loose wrist, it will naturally close and open up at the right point, when starting to work on this use putters and mid-range discs so you can do it slower than you're used to driving, and work on tightening your grip right when your wrist opens up.
Blake, your input?