|Now that's Magic!|
It relies on the parent having an "Ultimate Deterrent", i.e. a punishment that's so horrible, a child won't risk suffering it more than once, but which causes no physical, mental or emotional harm (e.g. the loss of something precious, including a single meal). It also relies on the parent actually using the "Ultimate Deterrent", as failing to do so results in a total loss of credibility.
Having explained to the child the "Ultimate Deterrent" that will happen if the child fails to comply with the request by a count of 3, the parent then gives the child time to comply by counting 1 . . . 2 . . . 3. If, by the time 3 is reached, the child hasn't complied, the "Ultimate Deterrent" is dealt.
Hopefully, the "Ultimate Deterrent" is so horrible, the child won't risk suffering it more than once. Job done!
See also 1-2-3 Magic: Effective Discipline for Children 2-12 (Google Books).