172 tucks under
I tried this once (at high altitude)
Its OK until you let the speed bleed off, then without any warning it goes into a cross between a flick roll and a bunt. Quite scarey but the lesson was simple, watch the speed but it does give a great rate of decent. Only way to get it better ROD is get in a stable phugoid at about 35kts.
Remember aircraft bite fools quickly.