Fly Guy,

Any of the breathing "scoreable" reactions mentioned in Chapter 4 of The Lie Behind the Lie Detector would be okay. It's not necessary that all reactons be the same. Blocking is not only the easiest, it is also the most commonly observed reaction, so it's a good choice.

If choosing blocking, I would avoid exhaling "hard," that is, I wouldn't make an effort to exhale more than I normally might while, for example, pausing breathing to listen carefully. It would also be prudent not to exhale for a certain number count for each control question. If every breathing reaction looks the same (for example, has a close to identical duration), countermeasure use may be inferred. One of the countermeasure "indicators" sometimes mentioned in polygrapher training sessions is so-called "cookie-cutter reactions."

As we note at p. 147 of the 4th edition of The Lie Behind the Lie Detector, blocking should be done after exhaling.