O2 sensor oscillations

[Bill the arcstarter]

OK, Here's a question:
 I read that, while in closed loop - the voltage on the O2 sensor is 
supposed to cross 0.45 at some particular rate.

 Are these crossings producted by "puffs" of exhaust going across, or is 
the ECU intentionally running lean-rich-lean-rich-lean in order to keep 
the avg mix (over time) "centered" on the ideal?

Thanks!
-Bill

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