It's a guard skill so it works when you successfully guard against an enemy.
I'd like to note that it is dependent on your connection so lagging will make it delay just a tad.
I've read that jumping before shielding helps a little. <<kinda works
If you purchase it on a character of the same class, (Explorer, Resistance, Cygnus, ect.) you should be able to move them to your new character via shared cash shop inventories. Unless your on Reboot, I think.
