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In your fuel distributor below each injector port is a chamber that is divided into an upper and lower chamber by a steel diaphragm. This diaphragm is pushed against a drilled port that extends down from where the injector line attaches. There is fuel at slightly below line pressure below the diaphragm and fuel at line pressure but volume regulated by height of metering pin moved by air meter disc in airflow housing. The O2 sensor gives its signal to the Lambda computer which alters a duty cycle (percentage of "on time" ) for an electric valve that controls the escape of fuel out of the lower chamber. If the duty cycle goes higher (as O2 sesnor voltage goes down indicating lean) more fuel escapes and the lower chamber pressure falls and the ports out to injector become less restrictive and the quantity to injectors goes up. If the O2 sesnor voltage goes up indicating rich the duty cycle is decreased and the fuel escape is less and the lower chamber pressure goes up and the port to inj. is more restrictive and quantity to inj. goes down. This is a continually oscillating cycle.

50% duty is approximately what you get when O2 sensor is disconnected or non functional (This is 45 degrees dwell on 4 cylinder scale on a dwell meter if that is what you use to measure the duty cycle of frequency valve controlling the lower chamber pressure).