Below was posted on the Family Tree Maker message board and thought it was appropriate to post it here as well. Duff certainly can explain things much better than I.
The new TreeSync™ feature will sync a tree between one instance of a tree in Family Tree Maker and one instance on Ancestry.com.
To start, you can upload your tree from Family Tree Maker to Ancestry by clicking Upload and Link to Ancestry from the Plan workspace. This will create and link to a new tree on Ancestry. If this is a duplicate of one that you already have online, then you can simply delete the old one.
Or you can start with your tree on Ancestry if you already have one there. In this case, you can download and link it to a new tree in Family Tree Maker by clicking Download from Ancestry on the New Tree tab in the Plan workspace.
Once the two trees are linked, you can then access and edit in either place. But again, you must start with a single tree in one system and upload or download to the other--creating a new tree in the other.
TreeSync will not synchronize an existing tree in one place with an existing tree in the other.
Although your entire tree is uploaded, only you can see information about living people even if you make your tree public. If you make it private, only you and those you invite can see any information in your tree.
I hope this helps.
Family Tree Maker