Here is an interesting capture. In its animated form you can see the relationship between the Oxygen sensor voltage & the injection Pulse Width.

As the O2 sensor volts fall (due to the presence of oxygen in the in the lean state exhaust gasses) the injection pulse width can be seen to increase in response. Commanded by the PCM the injection period is increased to swing the mixture toward the rich end of the closed loop cycle. Up to the point were the O2 sensor volts peak & the pulse width time is seen to reduce again as the closed loop process continues.

This tightly controlled cycle from rich to lean is essential for efficient catalyst operation & prolonged efficiency.