The original Bensen was powered by a high-revving 2-stroke drone engine with a small direct-drive prop, and as a result the required rotor to prop clearance could be achieved with a relatively short mast. I'm told that this put the thrust line of the engine on the original Bensen was pretty close to the vertical CG - as per the later Glasgow recommendations. Some of the later Bensen derivatives were fitted with geared Rotax 2-strokes fitted with gearboxes which allowed longer, more efficient props to be fitted, requiring a taller mast. Whilst the taller mast did have the effect of raising the CG, that was somewhat counteracted by the engine being lower relative to the thrustline due to the "cranked" nature of the gearbox and the engine thrustline moving up - and so a significant offset between the vertical CG and the thrustline was introduced. The original Merlin with a 582 and relatively light rotor blades had some interesting handling qualities - for example it would pitch significantly nose down when power was applied and pitch nose up when power was reduced.
In aircraft with a high engine thrustline relative to the vertical CG then unloading the rotor (eg by rapidly pitching forward and reducing "G") whilst at high power settings can result in the aircraft pitching nose down as a result of the pitching moment of the thrustline around the CG when the drag from the rotor reduces. This in turn can result in a further reduction in "G" resulting in an increasing pitching force and if uncorrected can have a very unhappy outcome. Most modern designs seek to address this by bringing the engine thrustline closer to the position of the vertical CofG and/or use of a horizontal stabiliser.
Homebuilt gyroplanes can be put together with a range of different engines and props and are operated by pilots of varying weights. When they are built they are subject to a "hang check" where the aircraft is lifted by the teeter bolt with the pilot and a typical fuel load on board to check the angle at which it hangs. The reference for the angle (eg mast or keel) and the acceptable range vary depending on the design. A gyro with, for example, a heavy pilot and a light engine/prop will hang nose down - meaning that in normal flight the rotorhead will be operating closer to it rearward limit of movement than it would be with a light pilot / heavy engine. That can becomes an issue when flaring to land as the pilot may run out of rear stick travel before the rotors are at the required pitch angle for the flare. Conversely with a nose-up angle the pilot may run out of forward stick movement limiting the speed of the machine. The normal adjustment for this is to move the position of the rotorhead forwards or backwards until the required hang angle is reached.