12 why did the tractor need decompression when it started in the past?

Early production of 12 horsepower tractors, basically did not install electric start. To start the diesel engine, you need to open the decompression with your left hand, hold the starting handle with your right hand and turn it by arm strength. At this time, the decompression is released, and the right hand continues to rotate the handle, relying on the inertia of the flywheel until the flywheel rotates the piston to compress the top dead center, and the diesel engine will start normally.

If decompression is not started, the intake stroke of the piston ends, the intake valve and exhaust valve are all closed, and the cylinder becomes a sealed space. When the piston moves from the bottom dead center to the top dead center, the air in the cylinder is compressed, and the pressure can reach 30~40 kg/cm2 (tractor, a specialized course for middle schools in Jilin Province). It is conceivable that if the diesel engine is not started under reduced pressure, can it be started normally?

I don't know if the answer is right, please forgive me.