You have to give a 40 min presentation about you and your former research work (bachelor thesis, internships, master thesis, etc.) to all the supervisors from the topics you applied for. This is followed by around 20 min of Q&A with all the supervisors.

You must focus on presentation and improve your sceintific knowledge. Be on time. Don't worry. If your research is near to position subject you will accept. Don't contact professor and be patient to addmision office answer you.