A more thorough explanation:

Traditio brevi manu is a legal term from Roman law. It means giving something to someone who already has it. This is called constructive delivery. It is different from other types of delivery because the person who gets the thing doesn't have to give it back and then get it again. For example, if you lend a book to someone and then decide to give it to them, you can just tell them they can keep it. You don't have to take the book back and give it to them again. This is an example of traditio brevi manu.