Thick line of separation
This line is often drawn using an arbitrary requirement, but the requirement need not be completely arbitrary; e.g. members must sign an oath of loyalty, profess their unwavering fidelity to the organization or complete acceptance of the truth of a particular set of doctrines, or wear in public (though not necessarily in plain sight) a symbol, tag, distinctive hair-styling, or particular clothing.
Although such stiffening of membership requirements may quantitatively reduce the power structure's support base by excluding many who would otherwise be members, those who remain will be disproportionately supportive of the power structure due to having consciously made the choice to "cross the line". Depending on the particular requirement(s) used to draw the line, members may also be specifically disinclined to question the power structure's motives or reasoning, thus helping to ensure the responsiveness of the support base to the power structure's wishes. As such, this can also be a form of asymmetric entry-exit.
Drawing a thick line of separation is a form of choice between extremes ("you're either with us or you're not").