Still trying to understand the auto-decompression ball bearing
This is what a pushrod valve system looks like:
As the camshaft rotates, the cam follower encounters the pointy part of the cam and is forced to move upwards. This in turn moves the pushrod and one end of the rocker arm upwards. The other end of the rocker arm moves downwards and depresses the valve stem, causing the valve to open.
When the cam follower is in contact with the flat part of the cam, then the valve is fully seated (closed)
But on that particular engine as shown here:
https://youtu.be/Y4Xd4-3Vjk8?t=426 the bottom part of the exhaust cam is not completely flat, there's that ball bearing protruding.
This will in turn make the valve lift just a tiny bit. It will vent some air during the compression stroke, making the cord pulling easier, but not enough to affect the combustion significantly.
Then, when the engine picks up RPM, the centrifugal mechanism retracts the ball bearing, making the bottom of the cam completely flat and allowing the valve to close properly.