I thought I had understood it but here I am in an unespected result :
I am emperor of france and I wanted to get lands out of the the holy roman emperor who has quite a large empire. He has as his vassal the king of aragon who happens to belong to my dynasty. I invited the Aragon heir (of my dynasty) to my court and pressed his claim on the throne, expecting him to become one of my vassal kings since he is family.
However when I won the war two things happened :
He was independant (I had pressed earlier another claim for a family meber and he became my vassal)
He only inherited half of aragon, the rest staying under the rule of the former king who became a duke vassal to the HRE
Can someone explain it to me ?