During takeoff and landing, cabin crew are secured in special jump seats and follow procedures designed to protect them in the event of an emergency.
One of those procedures involves adopting a brace position when instructed or when circumstances require it.
The exact procedure can vary by aircraft, airline, and emergency scenario. However, the goal is generally to position the body in a way that can help reduce injury during a sudden impact.
Keeping the hands tucked under the thighs can help keep the arms in a controlled position rather