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Mike a plane that will diverge downward can show that property safer when it's not plummeting.
If such a plane is permitted to get too fast, it can become uncontrollable/unrecoverable. Any incipient tuck is more easily seen in a moderate dive, and stopped before it gets too far.
I've had a glider that was unrecoverable due to tuck from a moderate dive. 3 times!
Last time too much damage to bother with.
I look for just a bit of tuck on some fast aerobatic slopers.. I use it as a speed limit. The recovery can place a severe stress on the wings when a plane tucks.
Otherwise a slight amount of nose-up from the moderate dive is what I look for.
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My buddy Mac and I were flying a Cox Lazy Bee ARF... the one with the zero pitch prop. Made lots of noise, but very little airspeed.
I coaxed it to a reasonable altitude and pushed over to get some speed for an Immelmann.. moderate dive. Pulled it up and rolled out at the top.. and then gave the xmtr to Mac for him to try.
To Mac, it's straight and level, or straight up/straight down.
He dove to pick up speed. Waaaaay too much speed, even for a LzB. It tucked! He pulled full elevator, and the wingtips touched, then the wings left.
We laughed all the way down... (It wasn't our airplane.)
We said... "Hey, Bob, your airplane is crashing!"...