This heavy duty Maillon rapide (Quick Link) is the accepted method of attaching a reserve parachute to your hang gliding or paragliding harness.
It is vital to make sure you use one of the correct quality and strength. To use a substandard link in your reserve would be foolhardy. Thus make sure you purchase a maillon that comes with the appropriate certs explaining its strength. (Like this one.)
What does it do?
Well it is essentially to do two jobs.
If your bridle was connected webbing to webbing then a shock loading (eg the reserve opening) can generate enough heat, as the webbing knots slide tight, to cut the webbing. This metal link prevents this.
It can be used to back up your main suspension system. Thus if correctly fitted one would not rely on the main karabiner or suspension system. (If your karabiner was left open or failed and your harness became detached from the glider you would still be attached by this link to your reserve.
How do I fit it?
It can be done up with a spanner. Avian on line recommend that you use threadlocking compound on the threads so that they cannot vibrate loose.
Avian, then recommend that you arrange the webbing such that the loading is in the direction of the greatest strength of the link.
Make sure you pack your chute and bridle so that it is comfortable and can not wear against anything.