If I had read the FAQs, I would have understood it. In general, I only learn about features of applications that I need. I do not read manuals. There are no little pop up "tips" or other ways for people to learn Geni features and this detracts in my opinion. A lot of older relatives have trouble using Geni.
As far as explaining the pushpin, I think it's simple -- if you want to know the relationship of two people, go to the first person, e.g., Thomas Jefferson, and click on the pushpin on the upper left (?) of his profile. Then go to the profile of the second person, e.g., your grandfather, and click on the blue rectangle "Show Relationship" in the upper middle of his profile. Geni will then show the relationship between Thomas Jefferson and your grandfather (or which two profiles you have used the pushpin with). Normally, when the pushpin is not "pushed", Geni shows all relationships with respect to you.
When you want to stop showing relationships in terms of Thomas Jefferson or whomever you've "pushed" the pushpin for, just click again on the pushpin of the first profile (e.g., Thomas Jefferson in this case) and Geni will default to showing relationships to YOU.