I think what he means is that in LEVEL FLIGHT the aircraft behaves a bit unusual: If you raise power, it will pitch down and loose height, if you reduce power, it will at first raise the nose and climb, until it is slow enough to sink... I also found it a bit strange at first, but it is most noticable with empty fuselage tank and full reserve (foward CG).
I have same experience with A2A P-40: if the speed drops, the nose rises. If the speed rises, the nose drops. This means, that it's absolutely impossible to trim
to fly level: either you set the nose too high, the nose rises more, speed drops, nose rises more, speed drops, stall, spin... or you set nose too low and speed increases, nose drops, speed increases, nose drops, dive and rip off your wings. No matter what amount of throttle and what amount of trim, no combination can keep the plane level even just for 30 seconds to read the map for example.
But I think this tendency is exactly the opposite to what you said: it's when fuselage tank is full, when it tends to behave this way. Which is why I use that tank first (leaving around 5-10 gallons in it because the plane should be landed with fuselage tank selected, IIRC, thus shouldn't be completely drained). With fuselage tank full, the plane is outright awful to keep stable in any speed, power setting, climb, level or descent. With CoG at rear, it's understandable that the nose will be higher for fixed speed and elevator trim. It's just this odd way to inverse speed to trim requirement baffles me. I thought that even with rear CoG, it would merely have a more nose-up
attitude but the nose would still drop when approaching stall, instead of the otherwise.
Then again, I'm not saying it's impossible to have this sort of behaviour. I think there has been some planes that actually did perform that way, like Wright Flyer for example: if you approached a stall, the main wings stalled first, and with it's elevator being positioned in front of main wings, the nose pitched up violently slowing it down further, and the aircraft became almost instantly unrecoverable due to the "nose pitch up near stall" tendency. Luckily these Wright Flyers didn't fly very high so they were able to walk away from the wrecks they all too often found themselves in.
EDIT: fixed nosedown to nose-up. Obvious mistake from the context but I fixed it anyway.
EDIT: "trim", not "trip"