I loved your question, because I had no idea what herbst and forsus even were let alone the difference. So I just had to find out and learn something new. This is want I was able to fine.
this was a study done on them
Background: The best known of the fixed functional appliances are the Herbst appliance and the Jasper Jumper™. The Forsus™ spring is a new development which has been tested in a clinical study to establish when it can be used and to compare it with its predecessors.
Patients and Method: The Forsus™ spring was used over a period of 4 months to treat 13 patients with an average age of 14.2 years with Class II malocclusion.
Results: Evaluation of the lateral cephalograms showed that dental effects accounted for 66% of the sagittal correction. The sagittal occlusal relations were improved by approximately 3/4 of a cusp width of the mesial on both the right and left side as a result of distal movement of the upper molars and mesial movement of the lower molars. Retrusion of the upper and protrusion of the lower incisors reduced the overjet by 4.6 mm. Intrusion and protrusion of the lower incisors reduced the overbite by 1.2 mm. The occlusal plane was rotated by 4.2° in clockwise direction as a result of intruding the lower incisors and the upper molars. The maxillary and mandibular arches were expanded at the front and rear during treatment. – Evaluation of a questionnaire filled in by the patients after 2 months of treatment showed that approximately half of them had experienced difficulties in brushing their teeth. The main problem, however, was the restriction experienced in the ability to yawn. Overall, two thirds of the adolescents found the Forsus™ spring better than the appliance previously used to correct their Class II malocclusion, such as headgear, activator or Class II elastics.
Conclusion: The Forsus™ spring has stood the test in clinical application. It is a good supplement to the Class II appliance systems already available
I also found this web site that has more information as well photos, I thought that should be able to help you know more about them
I hope this help you