Since you are familiar with manifold pressure let's start from there.
Different designs measure this in different units. Luftwaffe aircraft use atmospheres (or Ata), Usaf planes use inches of mercury (Hg) and Italian planes use milimetres of mercury.
The mercury thing is because of some of the first atmospheric pressure experiments conducted by Torricelli. A rough desciption is that if you take a soup-bowl, fill it with mercury and then put a lab tube in it making sure to keep it in a vertical position, the atmospheric pressure on the bowl's surface will force mercury upwards in the tube. This enables you to measure pressure changes if you add some kind of scale to the tube.
On a standard day (as defined by another set of conditions like temperature and so on) at sea level, the unit conversion goes
approximately like this: 1 Ata = 29.92 inches Hg = 760mm Hg.
So what does that have to do with boost? First of all, boost is also a pressure unit but it's measured in psi.
However, the most important difference is that while the above units are all absolute,
boost is relative.
Relative to what you'll ask. Well, relative to the ambient atmospheric pressure on a standard day at sea level. It shows you how many psi higher or lower than that you are running your manifold pressure.
In that sense, the unit conversion becomes: 1 Ata = 29.92 inches Hg = 760mm Hg = 0 psi boost
Now, let's put two and two together. First of all, an engine that's not supercharged will be limited by the amount/density of ambient air in how much power it can produce. This is reflected by the fact that as we climb, manifold pressure drops for a given throttle position...whereas 65% of the throttle's travel range might give us full take-off power at sea level, it might take 85% of the throttle's travel range to do the same at 5000ft and it will be impossible to maintain even with 100% throttle past a certain altitude.
Another implication here is that if manifold pressure is equal to the ambient air pressure, then we are running the engine at the maximum power it can produce without supercharging it.
Let's say we're taking off from an airfield that's only a few feet above sea level so the pressure differential is negligible. In such a scenario, ALL non-supercharged engines are limited to 29.92 inHG=760mmHG=1Ata of manifold pressure on their take-off run. What is that if we measure it in boost? Correct, it's ZERO psi boost.
This is exactly what i've been meaning to conclude the discussion with, zero boost is by no means low power. In fact, the higher you go the more you should be impressed if your engine can maintain zero boost, because it means you are running a supercharged engine that runs at sea-level power when you are a few kilometers higher than that.
There's two more things you might want to keep in mind. First one, just like you observed yourself, if you are running at a certain manifold pressure and RPM and then decrease the RPM by a large margin, you'll see that the MP increases. I don't really remember the explanation to this, but it happens in real life so that's good enough for me to have it in the sim
Second, the RAF used an automatic boost controller. This means that unless you change the RPM all over the place to "confuse it" (as per the previous point) at which point it will need readjustment, a Spitfire or Hurricane will automatically throttle up and down to maintain the boost you selected, regardless of altitude changes. If you maintain +5 psi boost after take-off and you don't change your RPM, you can leave your throttle lever where it is and you'll see that at 8000ft you'll still have +5 psi boost.
Since this was an automatic system and automatic systems need manual over-rides for safety reasons, they also introduced the boost cut-out mechanism. I don't really remember how this works in the mechanical sense of things (search for posts by Viper2000 if you want to know how it really works, i think he's an engineer by trade and he provides lots of very good information about these things) but it seems that in a case of malfunction at certain altitudes the pilot would be left unable to command full power, so they installed this as an over-ride mechanism to ensure full power was always available even if the auto boost control failed.
An unintended side effect was that if you disabled it, you could run the engine at higher MP and thus, higher power. So it ended up being similar to a war emergency power system.
As for the slow running cut-out, it's used to shut off the engines when idling on the ground.