What Knobs are for 
I'm sure you know what "pitch" is so we'll go from there. CCPM heli's mix pitch and throttle curves so that when you move the throttle stick the pitch also changes. In normal mode the pitch will increase as you increase throttle. If both the knobs are set at zero and you've set the pitch correctly in idle up mode such that you have 0 pitch at mid stick, then you should have a linear pitch curve (line) from -1 to +7. This means that at bottom stick (0 throttle) you have have -1 pitch on the main blades and at top stick (100 throttle) you have have +7 pitch. This knob is mainly used as a pitch trim (regardless of the name) so that if you can't get 0 degress of pitch at mid stick in idle up you can use this knob to move the pitch curve so that it is exactly 0. 

Now the hover pitch knob will move the entire pitch line up and down so I'll give you some examples (not the actual pitch settings). So in normal mode and hover pitch set at 0, you have -1 pitch (bottom stick), +3 pitch (mid stick) and +7 pitch (top stick), if you turn the knob clockwise (to the 3 oclock position) you'll make the pitch settings +1 (bottom), +5 (mid) and +9 (top). If you turn the knob counter clockwise to the 9 oclock position you have -3 (bottom), +1 (mid), +5 (top). 

In effect you've changed the pitch of the main blades at the different throttle positions. 

Now to the pitch trim knob, this knob adds a curve to what was previously a straight line. This may be done for various reasons but mostly it's so that you can allow the rotor rpms to increase faster than the pitch a low throttle (this allows for better tail control). You can also use the hover trim knob to do this but since it changes the low and high pitch settings you may not like that. For instance, if you increase the hover trim setting you'll probably bog down the motor by adding too much pitch when you don't have enough rpms. So turn the pitch trim knob clockwise and you'll end up with a curve with the start point at -1 (assuming the hover trim knob is set at 0) and the end point at +7 but there is now a curve to the line which would be below the straight pitch line. What this means is that as you increase the throttle (which is linear) the pitch won't increase at the same rate as the curve starts out fairly horizontal, it will increase slowly then faster and faster as the curve of the line starts to get more vertical (see picture below, the dashed line below the straight line). 

Now you can combine the two in order to change the entire pitch curve (except to make the range bigger, this is a function of how far out on the servo arm the link is attached, the farther out on the link the larger the range). 
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The pitch trim isn't that hard to understand, it's a curve instead of a straight line. So as you increase throttle the pitch doesn't increase linearly. 


Example: 

In normal mode I'm going to list the throttle curve above and pitch curve below for a decrease in pitch trim producing a bottom curve (like the right side of a U). 

0 - 25 - 50 - 100 (in percent of throttle) 
-2 - 0 - +3 - +8 (in degress of pitch) 

as you can see in the first 25% increase in throttle the pitch only increases 2 deg, in the next 25% throttle the pitch increases 3deg in the last 25% increase of throttle the pitch increases 5 deg. So the pitch increases slowly at first but faster later on. What this does is give you the full range of pitch (and the same low and high pitch settings) but increases in a curve not a line. This is a great tool so that you can allow higher rpms on at lower throttle stick compared to pitch, this allows in a higher tail rotor rpm which gives you better tail authority. The down side is that you have to be at a higher throttle position in order to take off and hover but still gives you the same total pitch range and start and end pitch determined by the hover pitch trim knob. 

If you used hover pitch to get higher rpms at low throttle position it would also reduce the bottom and top pitch setting. 

The hover pitch trim is similar to the expo function in a computer radio which affects how much the cyclic servos rotate for a given amount of elevator and ailleron input. 

If you really want to test this I'd suggest you take the heli, disconnect the motor, put on a pitch guage and see how rotating the knob affects the pitch at given throttle stick locations