I think the point is to get as close to center of the valve as possible thru the sweep and not at the edges. The diagram is exaggerated but correct in theory.
While that is what comp says, others have made some very convincing arguments as to why that is wrong.
What they have said is:
Push down nopt sideways on the valve.
If you are pushing (straight) down, it doesn't matter if you are on the center of the valve or the edge, down is down.
The narrower the pattern, the closer to straight down it is.
Bear in mind I really know nothing, other than the reading and talking I did prior to the one time I actually did this.
But, I have a tremendous respect for Mike Lewis as "someone who knows" and he talked to me on this subject on the phone, has written in a number of websites on this subject, and is backed up by some major "heavies." If you read threads on speedtalk about this, you will know the names.
Worth mentioning is the more lift, spring pressure, and rpm, the more important valvetrain geometry gets. Many of these guys are lifting the valves 3/4" or more, and doing it at ten thousand rpm!