The motion of the cam is transferred to the valves through rocker arms.
Valve overlap is the period during engine operation when both intake and exhaust valves are open at the same time. Valve overlap occurs when the piston nears TDC between the exhaust event and the intake event.
Valve overlap is the number of degrees of camshaft rotation during which both valves are open.
The sequence order of events in a four stroke engine is
1. Suction
2. Compression
3. Power
4. Exhaust
In petrol engines, during suction stroke, air and fuel is drawn in the cylinder.