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BLHeli passthrough means that you can flash or program your BLHeli ESCs using your flight controller's USB port, rather than having to connect the ESCs up to a toolstick or Arduino programmer. This is especially helpful if you direct-solder your ESCs, but it's really great for anyone who runs BLHeli to be able to quickly adjust your ESC configuration without having to take your copter apart.
BLHeli passthrough is also sometimes known as Cleanflight passthrough or Betaflight passthrough.
My previous video on this topic is one of my most popular ones, but it's a bit out of date, so I wanted to do an update.