They need a warm and cool side, so you don't want to trap all the heat in. If they're feeling chilly they'll huddle under the heat plate. Read their body language as well. If you notice they are all huddled together and loudly chirping they may be too cold. If they're spread out and panting, they're too hot.
Yes they go under the plate once in awhile but usually just walk around the edges of the plate since they are a lil bigger
That's what healthy chicks should be doing.
It really does work well. I have had two flocks use these and I really like them. They don't need to "hold" heat - as heat is direct when they are close to the panel radiant heater.
I think one must take factors into consideration: What time of the year is it?, How good is my security, and if I need heat outside, do I have the electricity to do so.
Think of it this way: does a broody hen keep her chicks in a heated area until they are fully feathered? Absolutely not. I've had broodies hatch chicks and when they were a few days old we got a snow storm. The air temps dropped into the 20s. The chicks were just fine. They would scratch around with mom until they were cold and would dart under her to warm up and she'd stop scratching.
