If the first and second stage turbines are mechanically coupled to each other, the system is said to be a direct-drive engine or fixed turbine. These engines share a common shaft, which means the first and second stage turbines, and thus the compressor and output shaft, are connected.
On most turbine assemblies used in helicopters, the first stage and second stage turbines are not mechanically connected to each other. Rather, they are mounted on independent shafts and can turn freely with respect to each other. This is referred to as a “free turbine.”  When the engine is running, the combustion gases pass through the first stage turbine to drive the compressor rotor, and then past the independent second stage turbine, which turns the gearbox to drive the output shaft. 
 

TRANSMISSION SYSTEM
The transmission system transfers power from the engine to the main rotor, tail rotor, and other accessories. The main components of the transmission system are the main rotor transmission, tail rotor drive system, clutch, and freewheeling unit. Helicopter transmissions are normally lubricated and cooled with their own oil supply. A sight gauge is provided to check the oil level. Some transmissions have chip detectors located in the sump. These detectors are wired to warning lights located on the pilot’s instrument panel that illuminate in the event of an internal problem.
MAIN ROTOR TRANSMISSION
The primary purpose of the main rotor transmission is to reduce engine output r.p.m. to optimum rotor
r.p.m. This reduction is different for the various helicopters, but as an example, suppose the engine r.p.m. of a specific helicopter is 2,700. To achieve a rotor speed of 450 r.p.m. would require a 6 to 1 reduction. A 9 to 1 reduction would mean the rotor would turn at 300 r.p.m.
Most helicopters use a dual-needle tachometer to show both engine and rotor r.p.m. or a percentage of engine and rotor r.p.m. The rotor r.p.m. needle normally is used only during clutch engagement to monitor rotor acceleration, and in autorotation to maintain r.p.m. within prescribed limits. [Figure 5-3]
Chip Detector—A chip detector is a warning device that alerts you to any abnormal wear in a transmission or engine. It consists of a magnetic plug located within the transmission. The magnet attracts any ferrous metal particles that have come loose from the bearings or other transmission parts. Most chip detectors send a signal to lights located on the instrument panel that illuminate when ferrous metal particles are picked up.