I'm not exactly sure how it determines how to switch, but it would most likely be due to the CPU utilization at the time. It's just built into the design of the chip, much like idle power states and hyperthreading.
Keeping your CPU utilization at 100% for each core is a good way to have it stay at 2.6GHz. But your battery life will suck.
Coming up with a quick car analogy, think of it this way. In your old car you only do 40-70mph most of the time. Occasionally you get up to 100+. You get decent gas mileage - about 300 miles to a tank. If you hold the pedal as far down as it will go, you max out at 200mph. Gas mileage drops to about 100-150 miles.
You get a new car and keep the same driving habits. You do 40-70 most of the time, occasionally doing 100+. You get much better gas mileage - about 500 miles to a tank. Holding down the gas pedal, the car does 260, max and gas mileage drops to 300-350.
Coming back to computers, when you're watching HD video, listening to music, surfing the web or writing a term paper, you're only using a fraction of the CPU's potential. Having it clocked down to 1.3 GHz is a good thing and will get you close to the stated battery time. When you fire up a game or start rendering audio/video/3D graphics, the computer will automatically ramp up to the turbo speeds to complete the tasks faster (this is called rushing to idle).
The chip works as hard and as fast as it can to finish the job so it can go back to sleep (reduced power stage). It's very complex, and something better left to the professionals at intel.
it will go to 2.6 when it is needed...if you are just browsing the internet it actually may be down at like 800MHz...turbo boost also underclocks...if you are at 100% on all cores, it may boost up to like 1.6...if you are at 100% on 1 core...it will boost all the way up to 2.6...it is to make single threaded apps run faster...and also the turbo boost cuts off around 90-95C to bring the heat down...just let it do its thing..there is nothing you can do about it...except crank up the A/C