Rule: One break in any 24-hour period must be at least 6 hours in length
Similar to the rule regarding the maximum number of splits for rest periods, this rule was introduced to ensure that a sufficient amount of uninterrupted rest/sleep was granted to crew.
Essentially:
Out of the 2 rest periods (qualifying towards the 10 hour rest period), given to a crew member in any 24-hour period, one of those must be at least 6 hours in length
The 24-hour period is not specified, so the rule can be calculated in any timeframe.