AeroTech has engineered a new machined anodized aluminum screw-on motor retainer. It is designed to fit all AeroTech kits with 29mm motor mounts and will fit other manufacturer’s 29mm motor mount tubes with an O.D. of 1.235"

Retainers are used in place of the hook. It makes it easier to install or remove the motor from the engine mount.
Once installed, the retainer doesn't require any tools to operate. Simply install the model rocket motor, and screw on the cap.