I will eventually add this to my handout (available in a previous blog post), but I don't want those who have asked me how to do this, to wait until I get around to it.
It works best if you start this at the end of a diamond (I'll post what happens if you don't at the end). Your threads should look like this, and you should have passed the weft from left to right.
Next you will flip all the cards on their verticle axis, keeping them in the same packs. This is equivilent to changing the direction that you roll the cards, but you can continue the motor memory that you have established because you will continue to roll away from your body. This will remove the twist that has built up.
Starts in this postion (or reversed).
Next you will take the cards at each end, and slide them into the opposite pack. There will be two cards next to each other at the outermost edge of your packs instead of one.