I have used the "My Feed" for several years and have lost count of how many things I have followed but over time I believe I have figured out how it is working.
When you search for something and see the results you have a choice right there to either watch or not watch the show or movie depending on availability which can either be "buy" , "rent" or "free". Once you follow you will be notified when there is a change in one of these statuses or if the item appears in a new channel where it wasn't before. So for example, if an item was previously not available or only had a buy option and is now available for rent you will be notified, or if it is now free then you will be notified. I use the feature mostly to wait to be notified for when movies and shows are free (and by free it may mean you have to watch it on a channel that shows commercials, which is fine by me as it's no different than free antenna TV.
This seems to work most of the time although at times it does seem a little random when it notifies you and on several occasions I have also found things that are not listed correctly. I currently have around 50 items that show up in my feed.