right seat, left seat
n.— «All deploying and redeploying units have an overlap time so the new unit can learn the battlespace from the previous one. The process is called “right-seat, left-seat.” New units coming into the area ride along with the unit they are relieving—sitting in the right seat to observe operations. After the familiarization, the new unit gets in the left seat and the relieving unit observes. The process takes a couple of weeks.» —“‘Artificial Spike’ Leads to Highest Troop Level in Iraq” by Jim Garamone Military Family Network Aug. 8, 2007. (source: Double-Tongued Dictionary)