Hey PJ,
That is an excellent question, and it's actually pretty simple how the brakes are applied when using the Curt Echo Wireless Brake Controller # C51180.
The Echo # C51180 has an accelerometer in the unit that senses when you are slowing down, and even the amount you slow down. So when you hit the brakes on your Subaru, the Echo will know and apply the trailer brakes based on how quickly you decelerate.
That's why it doesn't need the brake wire connected from the vehicle; it will... view full answer...
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