OK, I am new to this equipment so I have a lot of newbie questions. I re-read some previous posts again on sumps. I think I am starting to get it.
With a hole in the tank it can only drain by gravity when the water level is high enough. Therefore, power off - no flow to sump or tank, power on - flow to tank so overflow to sump.
With an overflow box, a siphon tube concept generally is not a problem as long as the siphon is not lost (when a power fail occurs or any other time when the pump is on). However, this normally does not happen.
Still not clear how the box works but here is my guess (please correct my understanding), water siphons from the inside box to the outside box and then overflows to the sump, sump pumps to the tank, level in outside box matches the inside tank level? If this is it, not sure why it would ever loose siphon unless the main tank drops below the siphon level of either side of the siphon tube??
Power off - no flow to sump or tank, power on - flow to tank so overflow to sump.