Intake:

As the piston moves upward and approaches 28° BTDC (before top dead center), as measured through crankshaft rotation, a camshaft lobe begins to lift the cam follower.

That causes the pushrod to move upward and pivots the rocker arm on the rocker arm shaft. Since the valve lash is taken up, the rocker arm pushes the intake valve downward and the valve begins to open. An intake stroke now starts when the exhaust valve is still open. Flows of the exhaust gasses will have established a low pressure condition inside the cylinder and will help pull in the fresh air charge as display in Figure.

The piston continues its upward travel by TDC (top dead center) when fresh air enters and exhaust gasses leave. At about 12° ATDC (after top dead center), the camshaft exhaust lobe rotates so which the exhaust valve will begin to close. The valve is fully closed at 23° ATDC. This is completed by the valve spring, that was compressed while the valve was opened, forcing the rocker arm and cam follower back against the cam lobe as it rotates. The time frame in during that both the intake and exhaust valves are open is known as valve overlap (51° of overlap in this instance) and is must to permit the fresh air to help scavenge (remove) the spent exhaust gasses and cool the cylinder. In many engines, 30 to 50 times cylinder volume is scavenged by the cylinder during overlap. This excess cool air also gives the essential cooling effect on the engine elements. 

As the piston passes TDC and starts to travel down the cylinder bore then the movement of the piston established suction and continues to draw fresh air within the cylinder.