It appears some people seem to be having issues with this, and I did as well... Here's some things to take note of.

1. By default it is Left Shift. I'm not sure about other schemes, but this is for WASD.
2. I do not believe you can remove a modifier key; you have to change it to something else to have another function use [modifier key]

How do you use it?

1. Hold [modifier key]
2. Press [example: attack key]

How do you set it?

1. Click on a control for a function [example: attack] like normal.
2. Hold down [modifier key]
3. Press [new attack key]

This opens up a new control strategy, for example you can have shield bash be [modifier key] + [defend key] for quick access, etc etc. I'm currently uploading a video that partially explains it, will link when done.

I guess I'll leave it at General Discussion since it's a new update and so on, will move to New Recruits section after a while I guess.

EDIT: Video link!