A herbie system is a syphon system with a backup. The gate valve controls how fast the water drains, which allows you to adjust the water level in your overflow box. You can the control if air is getting sucked into the pipe, or a constant suction of water with proper water height in your overflow box.
The emergency drain is placed at the top of your overflow box. You need one because if the main drain gets clogged or slowed, your return pump from your sump will still continue to fill the tank at a faster rate causing your tank to overflow. When plumbing your emergency drain, have the pipe above your sump water level, so if its getting used you can hear the falling water
