What is the function of a bypass pair?

Bypass pairs are typically used for branch HA deployments to provide both device and WAN circuit redundancy. If a device that physically terminates the WAN connection fails, the hardware bypass pair enables the ION to continue to bridge traffic even if it's physically powered off. As such, the other ION device can continue to forward traffic over that WAN circuit through the failed ION.  

Additionally, when the site is in Analytics mode or in Control mode, a bypass pair port type allow for the transparent insertion of an ION device between Layer 2 and Layer 3 devices at the branch. Thereby making proof of concepts and migrations minimally disruptive, requiring little to no configuration changes in the surrounding branch infrastructure.