The term “involuntary” used to describe Tourette syndrome tics is a source of confusion since it is known that most people with TS do have some control over the symptoms. Before tic onset, individuals with TS experience what is called a “premonitory urge,” similar to the feeling that precedes yawning.
What is recognized is that the control which can be exerted from seconds to hours at a time may merely postpone and exacerbate outbursts of symptoms. Tics are experienced as irresistible as a yawn and must eventually be expressed. People with TS often seek a secluded spot to release their symptoms after delaying them in school or at work.
Typically, tics increase as a result of tension or stress (but are not solely caused by stress) and decrease with relaxation or concentration on an absorbing task. In fact, neurologist and writer Oliver Sacks has described a man with severe TS who is both a pilot and a surgeon.
Fascinating section from the Wikipedia article on Tourette syndrome.
Link to Tourette Syndrome Association (UK).