A method of initiating and an apparatus for counting and/or disconnecting operations, particularly for printing presses, variations in pressure of an outlet nozzle arranged above a conveyor belt being evaluated for the contact-less evaluation of sheet products or the like moved on a conveyor belt and converted by a pneumatic-electric transducer into electric pulses, which comprises the steps of directing a jet stream against a perforated disc moved in synchronism with the machine and comparing variations in pressure head of two outlet nozzles, then feeding only the result of the comparing step to a pneumaticelectric transducer, is fed and is evaluated by a second pneumatic-electric transducer the result of the variations in pressure head in the first outlet nozzle.