If there is an opportunity to get more performance, there is left over power, and the component is within safe operating conditions - then the boost will kick in.
Then it will reside when that power is no longer needed or if it isn't safe to do so (I.e. excessive temperature).
In NVIDIA's words:
“Starting with our GTX 600 series and later, every application and game runs at a guaranteed, minimum base clock speed. If there’s extra power available, a Boost Clock is enabled, increasing clock speedsuntil the graphics card hits its predetermined Power Target. This dynamic clock speed adjustment is controlled by GPU Boost, which monitors a raft of data and makes real-time changes to speeds and voltages several times per second, maximizing performance in each and every application.”
It will often come down to if there is safe operation for it to boost, then it will boost. Basically if it has extra power available and the temps are within safe ranges, it will boost to your boost clock speed.