When breaking out the first connection following Milling operations, a Manual Tong was latched onto the Drill Pipe to secure it in place. The bottom connection had already been broken and was resting in the thread housing from the below Drill Pipe. The Manual Tong was slipping on the Drill Pipe when force was applied through the Power Swivel so the Driller instructed the Floor Man to stay within the Red Zone to hold it in place.