Boost is a measurement of manifold pressure. Boost goes to zero when the throttle is closed becuase the motor swallows all the air in the manifold. If you shifted a turbo car with your foot flat on the floor, boost would not go to zero on shifts. You'd also likely over rev and blow up your car.
With a stock turbo and stock converter car at idle, if you floor the car you can count 1,2 and the turbo will spool. Once a turbo spools it heads straight for max boost as fast as the needle can swing. By power braking a car against the converter, you can get to the spool point and still be standing still. When you release the brakes you leave with almost instantanious power (zero lag).
This is very hard to do on a manual car. The torque converter allows an automatic car to store energy before launch, while the manual cannot without smoking the clutch or using some type of electronic rev limiter which come nowhere near "storing" the amount of power a converter can.
As far as cruising around goes, the higher the RPM you are running, the closer you are to instantanious boost. At 65 mph on the highway if you toe it enough to down shift into drive, boost will be right there. If it doesn't downshift or the converter stays locked, it will be lagging until the turbo spools.
The idea of using nitrous as a way to get the turbo to spool quicker with a manual is silly. Suffice to say the first weekend you get it to work you will break your rear end in two, if you don't smoke the clutch first. You aren't the first person to have tried this, Some have spent 10's of thousands of dollars trying to make it work. It's kind of funny how out of the 40K+ turbo regals built and the hundreds of shows, magazine articles, etc, you never see any successful manual trans, stock 3.8 Block powered cars.
Kinda makes you stop and think, doesn't it?