Synchronous does not require everything to be locked down. Unlike most history based systems where it has been pounded into your head to fully constrain stuff or bad thing will happen (and they often do), Synchronous does not require it because all edits are localized to just what you have selected to modify. Now, the interesting part. SE has 3 types of constraints that will drive other unselected faces to move when modifying selected faces... Live Rules, Face Relations, and Locked Dimensions.
Live Rules - Used to dynamically relate those common face orientations one would logically "assume" like adjacent faces remaining Perpendicular in a rectangular box or a series of holes remaining in a line. These can be tailored to suit the desired change if there is an exception to this "logical" assumption.
Face Relations - Used to temporarily make two or more faces re-orient to each other, or it can be made a "persistent" relationship. Example... Make one face parallel to another. By default it is temporary but can optionally be made to "persist".
Locked Dimension - Dimensions by default are not locked and thus will either temporarily move geometry to a location that you type in or allow you to dynamically move a face and they will update with the new value. A Locked dimension will permanently remain the value specified until your type in a new value.
In my mind, the best practice would be to decide what things are critical to the design and lock those down with persistent face relations and locked dims and leave the rest to live rules, but you REALLY need to think about what is CRITICAL because it is easy to say "everything" but in reality it's just a few things.
Ken G.
