The quiet period is company policy rather than a rule for ordinary results, adopted to avoid selective disclosure. During it management skips conferences or speaks only to published material, and insiders are usually barred from trading.
A separate statutory quiet period applies around an initial public offering, restricting what the company and its underwriters may say. Both reduce the information flow, which is why volatility often builds into a print.
Example: Northwind Tools enters its quiet period two weeks after quarter end. An investor conference appearance is cancelled, and the absence of any commentary for five weeks widens the range of analyst estimates.
Related: earnings-report, pre-announcement, earnings-call, form-s-1, insider-trading