2nd curtain sync with a Canon 5D

aka "Merlin's Rockstar Flash" aka rear curtain flash aka 2nd curtain flash

1. Hit Menu
2. Scroll down to custom functions, hit select
3. Change custom function #15 from a 0 to a 1 to enable 2nd curtain flash
4. Hook up a flash (if using a 550/580EX, change setting on your flash to enable 2nd curtain, if using a 420/430EX camera controls it)
5. Change shooting to Tv mode, slow down the shutter speed to 1/30th a second or slower (it won't work at faster shutter speeds)
6. Take photo, flash should fire at the start of your photo and again at the end, producing a cool effect of half blurry with some sharpness captured by the flash.

(I'm writing this down because it took an hour of researching down ratholes to find it and I keep forgetting the entire process)