"When the seat is up, it is locked. When the seat is back, it's not locked. In the event of an emergency, an unlocked seat has more force during the impact, and the thrusting forward of that seat can cause passenger injury," said Brian Manning, a flight attendant for Mesa Airlines.
"Since most accidents occur during takeoff and landing, it is for the safety of everyone onboard the aircraft to have all the seats in their upright and locked position, added Mesa.
True. People sitting behind the seat that is reclined or unlocked will not be able to brace properly as well.
Even when the airline is being at rest you should always put your seats in their upright position. Why? Because anything can happen. Take for instance this Delta Airlines. The Greek god Zeus commanded the lightning to strike it.
Fortunately, everyone onboard was fine. But regardless, that's a reminder worthy to keep.