Istikhlaf in Prayer : Imam appointing somebody from congregation as the imam.
The imam of Al-Rajhi Mosque in Riyadh Sheikh Saleh Al-Habdan had some throat problem .He withdrew and was replaced by the muezzin. This is called Istikhlaf in prayer. A Very rare Scene.
The Istikhlaf in Prayer can also occur if the wudu of imam gets broken while leading the Prayer .
Also read Incident when 22 Muazzein became shaheed to complete one Adhan in Kashmir