It isn't one size fits all. The adult myobrace line comes in regular or large sizes, but other activators come in a range of shapes and sizes. If you want expansion, you need channels that are slightly wider than your current dental arches. I haven't been able to find a size broader than my current intermolar width just under 40mm. So I haven't gotten any noticeable intermolar expansion from the myobrace, but I have seen some slight lower intercanine expansion, my arch symmetry is improved, and my lower incisor crowding is totally resolved. Its ramp-shaped walls guide the teeth into alignment based on the bite force, and hold the mandible close to class I occlusion. There are several threads on the topic and a case report in the assorted stories thread. I've previously posted a list of links about myobrace and other activators that I can track down if you are having trouble locating the relevant conversations.

How does myobrace work for someone with class 2 malocclusion? Would it be a bad option for someone with an overbite? I worry about it pulling my maxilla backwards.

Posted by: JuliusO

How does myobrace work for someone with class 2 malocclusion? Would it be a bad option for someone with an overbite? I worry about it pulling my maxilla backwards.

I just answered this same question in PMs from someone with class II malocclusion. I'll paste some of my comments from that conversation here:

The question about exerting a backwards force on the maxilla is one that I’ve worried about myself. For example, mandibular advancement devices (MADs) used to prevent sleep apnea jut the mandible into a class III position (farther than activators like myobrace) and Dr. Mew warns against them because of the reciprocal backwards force on the maxilla. The only thing I can say is that I don’t feel my maxilla being pulled back and I don’t think my maxilla has become any more retruded in the 5+ months using them, but I’m starting with class I occlusion, so the activators don’t shift my mandible much ahead of where it always sits. I guess you’d have to see what it feels like in your case.

If the appliance is holding the mandible out farther than its comfortable resting position, then the mandible might pull backwards on it. Since the channels for the upper and lower teeth are attached, this backwards force would be translated to the maxilla. However, if your mandible sits comfortably in the position that the appliance puts it in, and it is normally held back in a class ii conformation by occlusion, then it shouldn’t be an issue. In that case the mandible wouldn’t be pulling back on the appliance so the appliance wouldn’t pull back on the maxilla.