Most of the housings support focusing by using mechanical gear on a lens and control by turning a small knob on the left hand side of the housing or port. With this type of underwater housing, the left hand can not hold the housing and the right hand has to be under strain leading to less sensitivity on the right index finger to operate the shutter lever.
The INON unique MRS (Magnetic Rotary System) port uses magnet ring on a lens instead of a mechanical gear and focusing is controlled only by rotating a "MRS Control Ring" on a lens port by magnetic attraction. The "MRS Control Ring" turns the focus ring on a lens at equal rate where we can control the lens with same gear ration/torque as like controlling the lens on land. When controlling the MRS Control Ring, the left hand holds the lens port from underneath, which helps to avoid overload on the right hand. Holding the camera body by the right hand and controlling the lens barrel by the left hand is exactly same style as shooting on land.