General rule is that loads to 750kgs don't need brakes, but loads over 750kgs do.
The brakes are attached to the tow hitch and as you brake the trailers momentum pushes into the back of the car tow hitch and compresses the hitch brake mechanism which in turn applies the brakes on the trailer.
Brakes are undesirable on a boat trailer because they often sieze up with the ingress of salt water, but it's a legal requirement.
Hope that helps a little.
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