Reduces the cooldown of your Mind Blast spell by X sec., and while in Shadowform your Mind Blast also has a Y% chance to reduce all healing done to the target by 20% for 10 seconds.
Improved Mind Blast is a priest talent that reduces the cooldown of the Mind Blast spell by 0.5 sec per rank, up to 2.5 sec at max rank 5, and while in Shadowform gives it a 20% chance per rank to reduce all healing done to the target by 20%, up to a 100% chance at ma rank 5. Mind Blast normally has a cooldown of 8 seconds, and with 5 points in the talent that is reduced to 5.5 seconds.
Mind Blast, though one of very few ways for a Shadow priest to deal burst DPS, is very mana inefficient compared to Mind Flay and DoTs. With that in mind, this talent is geared more toward PvP, where burst DPS is king - a shadow priest looking to maximize his sheer kill power in PvP will find this talent very useful. It's much less useful in PvE, however, where Mind Flay and DoTs will make up most of the priests damage. However, remember to consider that 4 points in this talent will allow exactly 2 Mind Flays between cooldowns.
Patch 3.2.0 (04-Aug-2009): Redesigned. Now, in addition to reducing the cooldown of your Mind Blast spell by .5/1/1.5/2/2.5 seconds, while in Shadowform your Mind Blast also has a 20/40/60/80/100% chance to reduce all healing done to the target by 20% for 10 seconds.