At a cruise (constant throttle) condition, the engine does not produce enough exhaust gas at a fast enough velocity to spin the turbine at a speed that can produce boost. This is what is termed "low load", ie minimal work being done by the engine. The engine is actually running under vacuum (just like a N/A engine).
When you start to accelerate (increase the load on the engine), the volume and velocity of the exhaust gases increases, causing the turbine wheel to spin faster, thus producing boost. For example, if you are driving at constant throttle setting on a flat road, there is no boost. When you come to an "up" hill, the engine needs to do more work to maintain the road speed. The increase in the load on the engine causes the turbo to produce boost.
Under constant throttle, boost is independent of revs. If you could "cruise" at 7000 rpm, you would still have no boost.
Maybe you should go to a website such as howstuffworks (I think that's what it's called), and study how a turbocharger actually works - lot simpler than us being here writing a "War And Peace" article on how it all works.