Simple, yet brilliant.
Here is a bit of clever engineering. This tow hook utilizes no moving parts except for a servo pushrod. As you can see in the pictures, the servo push rod traverses the center of the tow hook body and acts as a retractable pin upon which the tow line is captured. A second pin is pressed through the tow hook housing perpendicular to the push rod and prevents the tow line from slipping off. When the tow release servo is activated the push rod is drawn into the tow hook housing and the tow line is free to exit the hook.
This very compact, linear design is also perfect for gliders with very slender noses.