The double retainer ("bionator") covers the surface of both the upper and lower teeth and is held in place when the patient moves their lower jaw forward to bite into it. For maximum benefit, it needs to be worn for 14 hours/day for several months until the overbite is reduced. After that, it is worn only while sleeping, just like a regular retainer, in order to maintain the improvement that was achieved.
The goal is to encourage the lower jaw to grow in a more forward direction. If the upper teeth are leaning forward, it can help with this, too.