I'm thinking if this at least works on activated skill attacks and with a decent amount of crit hit chance this could go a long ways in crowd control (even on higher difficulty levels when the enemies are more resistant).
Pretty sure it's only on the use of the skill. Unless you have massive spirit regen and a high attack speed, this move's a waste. The spirit cost of the spell, half your spirit, does not provide enough survivability to make it worth while, especially in inferno. The monk has much better, and cheaper skills for crowd control and survivability.
Might be useful on monks who's build rely on large amounts of dex, fast attack speed, and life on hit. I'm just a straight tank monk so I'm not sure, but it seems to me that the spirit cost would still be much too high.