Tach was used in the engine log for checks etc.
Fairly sure that is not the proper way to log engine time. As has been pointed out,cruising a bit slower ie: maybe 2250 instead of 2350 (PA28's?) results in a reading not consistent with the "normal" way ie: Take off-Touchdown, the result is (to the benefit of the owner/operator) longer time between checks, but cheaper for the hirer, but, not I believe legal for engine log entry's (in UK)