Originally Posted by Blizzard Entertainment
Mind Spike
The idea behind Mind Spike is that you can't always settle into your normal, and high-ramp up rotation. It's also useful when you have to move or get school locked. (Source)
[...] We've seen some confusion about Mind Spike. The intention is that you can’t always get your full DoTs up on a target in time before it’s dead. Shadow priests sometimes aren’t sure what to do on, for example, a fight where a boss suddenly spawns an add that you’re supposed to quickly burn down and then go back to the main boss. Mind Spike is what you do to that add. As a rule of thumb, if it’s going to die in under 15 seconds, then go with Mind Spike. Otherwise get your DoTs up and go into your longer rotation.