There's no magic here, guys! Once you use an EX-ability you will gain the BUFF "EX-something". It lasts for 6 (six) seconds and it also has a visual effect - 2 green swirls around your bloodline.
Usually your ex will cast normally and you will never see the buff. However, if it gets interrupted you will notice it, and you can cast ANY Ex during those 6 seconds. However, you cant use the normal version of an attack if you got the EX-buff.
So, if you wanted to do Javelin(EX-m2) on an SM and you get interrupted and immediatly after wants to avoid a huge incoming nuke using your (regular) space, you'll cast Tempest instead!
This could really *peeeeep* you over, se be careful