How a turbo works (simply stated)
The reason you can't build boost while the engine is not under load is that you are not generating heat. If you are holding at X RPMs (waiting to launch) your engine is generating very low power (not consuming much air or fuel). It is true that exhaust gas is flowing through the turbine, but because you are not consuming much air or fuel, you are not generating much heat (expands the exhaust gases).
Think about it this way: if you were to put a little fuel and air into each cylinder and ignite it, the mixture will burn, heat up, and expand to a larger volume. This volume creates flow through the turbine, which spools the turbo. Because not much exhaust (expanded volume) is flowing, you will not build much boost. That's what is happening with an unloaded engine.
Now let's look at a loaded engine: if you were to put a lot of fuel and air into each cylinder and ignite it, the mixture will burn, generate a lot of heat, and expand to a much larger volume. This large increase in volume creates a large flow through the turbine, which spools the turbo (generates lots of boost).
By riding the clutch you can put a load on your engine and create boost. Or, you can quickly rev your engine and use the rotational inertia of your flywheel to generate a load, but this load depends on the rate of change of RPMs (how fast you rev).
Hope that helps.
