In the right circumstances, it is certainly possible to communicate without using signs (and in particular, it is possible to convey a volition, backed by a threat of force, just by saying or writing nothing).
Such actions, then, cannot be composed of a volition and the motion that is willed, for the relevant volition is absent (more on volition below).
For in the case of convulsive motion, there is no volition that one's limbs move; indeed, if there is volition in such cases, it is usually a volition that one's limbs not move.
On this view, volitions are the source of agency: an overt movement is an action just in case it is caused, in the right way, by a volition.
Locke assumes (reasonably) that in order for an action A to be voluntary, it must be caused (in the right way) by a volition that A occur (or, as Locke sometimes puts it, by a volition to do A).
Locke's view, then, is that an action is voluntary inasmuch as its performance is caused by a volition.
For example, Locke would not count the motion of my left arm as voluntary if it were caused by a volition that my right arm move (or a volition that my left arm remain at rest).
What is requisite to arrest a volition or retard the impulse of an existing passion is a contrary impulse.
The agent wills his arm to move or produces a volition that his arm is to move, and it is this mental willing or volition that is aimed at causing his arm to move.
