Dear somasimple,
Mobilisation of Wikipedia reference-linkfacet joints is a movement intended to bring about the release from tightness of any spinal joint involved in a protective response . The nature of the protective response is that the smallest groups of muscles will increase in tone to hold still the facet joint involved in order to reduce movemnts generally and begin a cascade that ultimately will be revealed as painful. It is not necessary to provide movement to facet joints such that only that structure is move and absolutely nothing else is moved , pushed or pressured in some way. To assume this would be necessary would be to miss the point of mobilising.
When appropritely trained hands apply passive movement effects to facet joints , it will be obviously necessary to exert pressure through muscle and other soft tissue. Sufficient pressure to cause movement however does not usually require a lot of effort. It is common however for my students to feel fatigue in their thumbs untill a certain amount of strength has been aquired.
Further on this subject as I am able
Cheers