A transmission system for transmitting power generated by an engine of a vehicle, comprising a power take-off for driving an implement and a hydrostatic unit for transmitting power from the engine to the power take-off, wherein the hydrostatic unit comprises: - a first hydraulic power unit wherein the first hydraulic power unit is a hydraulic pump and having a first connection line and a second connection line; - a second hydraulic power unit wherein the second hydraulic power unit is a hydraulic motor having a first connection line and a second connection line; - a valve for connecting the first hydraulic power unit and the second hydraulic power unit, the valve being positionable at least in a first position, in a second position and in a central position, whereby: - in said first position, the first connection line and the second connection line of the first hydraulic power unit are connected respectively to the first connection line and to the second connection line of the second hydraulic power unit, - in said second position, the first connection line and the second connection line of the first hydraulic power unit are connected respectively to the second connection line and to the first connection line of the second hydraulic power unit, - in said central position, the first hydraulic power unit is disconnected from the second hydraulic power unit; and characterized in that the hydrostatic unit is configured so that when the engine is turned off and the valve is displaced in the central position, displacement (V P) of the hydraulic pump is set to zero, thereby allowing a shaft of the power take-off to be rotated by hand; or wherein the system further comprising a planetary gear train and a clutch for selectively coupling the engine to the planetary gear train, wherein the hydrostatic unit is configured so that: when the engine is working and the valve is displaced in the central position, displacement (V P) of the hydraulic pump is set to a maximum value, and the clutch is disengaged allowing a braking action on a shaft of the power take-off. - a first hydraulic power unit wherein the first hydraulic power unit is a hydraulic pump and having a first connection line and a second connection line; - a second hydraulic power unit wherein the second hydraulic power unit is a hydraulic motor having a first connection line and a second connection line; - a valve for connecting the first hydraulic power unit and the second hydraulic power unit, the valve being positionable at least in a first position, in a second position and in a central position, whereby: - in said first position, the first connection line and the second connection line of the first hydraulic power unit are connected respectively to the first connection line and to the second connection line of the second hydraulic power unit, - in said second position, the first connection line and the second connection line of the first hydraulic power unit are connected respectively to the second connection line and to the first connection line of the second hydraulic power unit, - in said central position, the first hydraulic power unit is disconnected from the second hydraulic power unit; and characterized in that the hydrostatic unit is configured so that when the engine is turned off and the valve is displaced in the central position, displacement (V P) of the hydraulic pump is set to zero, thereby allowing a shaft of the power take-off to be rotated by hand; or wherein the system further comprising a planetary gear train and a clutch for selectively coupling the engine to the planetary gear train, wherein the hydrostatic unit is configured so that: when the engine is working and the valve is displaced in the central position, displacement (V P) of the hydraulic pump is set to a maximum value, and the clutch is disengaged allowing a braking action on a shaft of the power take-off. - in said first position, the first connection line and the second connection line of the first hydraulic power unit are connected respectively to the first connection line and to the second connection line of the second hydraulic power unit, - in said second position, the first connection line and the second connection line of the first hydraulic power unit are connected respectively to the second connection line and to the first connection line of the second hydraulic power unit, - in said central position, the first hydraulic power unit is disconnected from the second hydraulic power unit; and