First I will clarify how SS works, when you cast the spell on someone they gain a 30 second buff. Nothing happens until the target takes damage, when they do they will take the full amount, but will gain a second buff. This new buff will absorb the next 500 + 75% of your Spell Power damage and then expire, while it is active it will also increase the crit rate of FoL by 50%. This 2nd buff lasts a maximum of 6 seconds, and can only occur every 6 seconds.
This is most effective on Tanks since they take damage often, but can also be useful on group members if you know they are going to be taking damage more often then every 6 seconds. I am excited for this spell, and it will help smooth out burst on the tank.