Key Digital ES381 - Manual (Page 21)

Period mode should be used if you have a single pulse or a slow
input frequency. In the latter case, by timing the period from edge to
edge allows a very accurate measurement of low frequencies by
simply inverting the period measured. This is known as reciprocal
mode. The software shows this reciprocal.
Elan Digital Systems Ltd. 21 USBcount50 USER’S GUIDE Iss1