the key that Jon is assuming you understand here is that the "vertical feed" is sorta like what is called "walking foot" today. That is why it has two feet. One is part of the feed for the fabric. The needle itself does the rest, like rowing a boat. paddle in, move forward, paddle up. in this case needle in, foot holds, needle and food move back taking fabric, needle and then foot lift up and move forward.
This machine was very revolutionary. it is Totally undervalued in the market. A complete Mother of Pearl covered cabinet version is listed for $425 and still no takers after 3 weeks....
The fact that this is your Grandmothers is where the TRUE value lies. Several of us would give quite a lot of our very rare and expensive machines for a chance to have the machine that an ancestor sewed on