not gonna answer your question directly, but stay as far away as you can from the reaction manager. It's buggy and unpractical. Try setting up script/expression controllers instead.
Usually, what I do is i create a layer of animation with the poses i want via controller tracks, and make the control objects "wire" the weight of these tracks
for your jawbone, it would be:
2-open curve editor
3-setup a "euler list" controller in the rotation track of your slave object (jaw bone)
4-add a "euler xyz" controller to the available track of the controller list
5-go to the motion pannel, click on rotation
6-set the second track as "active"
7-open the jaw as far as it should
8-back in the curve editor
9-unfold the "weight" property of the "euler_list" controller
10-on the second track, add a float expression/script, whatever you're more comfortable with
11-add the "position_z" track of your control object as a variable ("z" for the exemple)
12-setup the right expression (typically "if (z<0,z*someValue,0)"). the goal is that when the control object is animated to the maximum, the value of the weight track is 100.
This might sound a little complicated, and maybe unclear (don't hesitate to ask more if you're interested and can't understand a step), but this is an extremely powerful method when you have to deal with complex facial rigs in max.
Sorry I couldn't be of more help on your actual problem though.