Intermittent indicators tell the solver to select or remove intermittent letters in the fodder (the part of the clue the indicator works on).Some intermittent indicators tell the solver to select even letters, some indicate to select odd letters, some work either way.
For example, phrases like 'evens out' and 'odds of' indicate to remove the even letters (or select the odd letters).Phrases like 'every second' and 'odds cut' indicate to remove the odd letters (or select the even letters).Phrases like 'every so often' and 'periodically' work for either.
"Singers regularly ace hook in recital (5)" -> 'regularly' indicates to select regular letters, no specification between odds or even. "ace hook in recital" gives us CHOIR.