As the cam has rotated another 15 degrees or so, we can see where the point of contact between the cam lobe and the shim/tappet is: WAY out there near the edge of the shim. As the cam rotates, the point of contact will follow the lobe from this side of the shim to the other side. Then as the lobe rotates to the point that the flat part of the closing side of the lobe is against the shim, then the point of contact will return to the middle of the shim. And then the valve will close.

The shift outwards of the contact point speeds up the valve movement. This is the fastest way to open and close a valve: the mushroom effect.

It is the fastest way to open and close valves SMOOTHLY.