An electric over hydraulic unit is a self-contained electrically operated pump/hydraulic reservoir which sits on the trailer (normally mounted centrally at the front of the trailer tongue behind the coupler).
When the in cab proportional controller detects vehicle braking, it sends a signal to the electric pump which pumps hydraulic fluid to the brakes at the correct pressure to match the tow vehicles deceleration.
Electric over hydraulic controllers are commonly becoming mobile and easily transferred between tow vehicles. They simply plug into the cigarette lighter socket, self level and calibrate themselves and can be positioned on the dashboard or drivers lap as required. They normally have all the bells and whistles of the top end fixed controllers with the convenience of being able to use multiple vehicles to tow the trailer.
With all trailer based electrical equipment which predominately operates at 12 volts, check your tow vehicle voltage output matches the equipment you are purchasing. If you vehicle runs 24 volts, it would be prudent to check and take advice from your local auto electrician before installing 12 volt equipment.
The great thing about using an EOH hydraulic braking system over electric drum brakes for trailers is that you have all the control, response and stopping power of electric brakes, but with the advantage of lower maintenance requirements and the ability to use the trailer brakes in both wet and marine applications.
EOH braking systems are expensive to purchase and install, but if you are planning on keeping the trailer for an extended period of time, do a lot of long distance hauling or everyday traveling with full loads, have limited downtime for maintenance or have a heavy trailered boat, then EOH brakes are highly recommended.