So, I'm to understand that the force aura is a force user's natural defense against the force.

Meaning, if someone weak in the force were to try to manipulate someone powerful in the force, it wouldn't work, regardless if the powerful force user were actually trying to defend against it.

How exactly does it work? I see cases of people like Obi Wan being able to throw Darth Maul with a force push, but other force users not even being able to phase Maul.

Or that instance where Savage Oppress force choked both Count Dooku and Asajj Ventress but could not kill them.

Can a force user increase their own force aura willingly?