This may be another person holding the aircraft for you, or by some kind of security device that you arrange. Any adequate retainer will work. Don't try to hold the aircraft and start the engine by yourself. Your presence at the field, and the use thereof is at your own risk. Though flying at the field alone is at the discretion of the pilot, it is recommended that an observer be present to aid the pilot while flying and during ground procedures.
Observers can help pilots in many ways.
No dangerous flying is allowed. No dives at the flight line, or flying over the pit or the parking areas. Flyers should occupy a flight station when flying. This basically applies during times where two or more persons are flying. During those times of student instructions or isolated flying such as the testing of an airplane, the flight station location may be waived.