Yes, I also have this tool and I've used it a few times. Once you remove the horse shoe clip holding the guide in place, you use the forks in the closed position on the guide.
It's been years since I've used it, but if I recall correctly, the bottom fork rest on the top of the lifter boss and the top fork goes between the spring and catches on of the ridges of the valve guide.
When the forks are "closed", that round-ish metal ram on the end of the rod slams into the bottom portion of the forks, forcing the forks open and pushing the guide assembly out the top of the block.
It takes a few whacks with the weight, put the tool works quite well. If I recall, it was made by All-Power. All-Power also made a Chinese-finger type of tool that used gripping power and brute force to pull the horse clips out really easily. I'd assume these tools were sold as a set. I also have that tool and it works very well.
Juergen, I re-read your post. Are you talking about the small flat area on top of the forks? I've used that to help position the forks and using a hard rubber mallet, drive them onto the guide.
Be mindful where you are using it. A few times, the valve assembly came shooting out of the block with some force. You don't want to cause any dings in any sheet metal near by.