Complex sentence: The 1st and 2nd SS Panzer divisions were to wait until 3rd SS Panzer Division attack had destabilised the Soviet positions at Prokhorovka; and once underway, the 1st SS Panzer Division was to attack the main Soviet defences dug in on the slopes southwest of Prokhorovka.
Split into simpler sentences: The 1st and 2nd SS Panzer divisions were to wait until 3rd SS Panzer Division attack had destabilised the Soviet positions at Prokhorovka. Once the Soviet position at Prokhorovka was under attack from the rear, then 1st SS Panzer Division was to attack main Soviet defences dug in on the slopes southwest of Prokhorovka.