The stock probe is a narrowband, it only indicates rich and lean inside of a limited band to the ECU. 0v is lean, 1v is rich, .5v is 14.7. This is only relevant when cruising though since the ECU doesn't read the stock O2 sensor under hard acceleration or greater than about 18% throttle.

Narrowband is basically bouncing between rich and lean all the time, it doesn't necessarily hold .5v but goes between 0v and 1v. As an example the volt to AFR could be 1v = 14 and 0v = 15.4, thus why it's called a narrowband. It adds fuel and removes fuel to average 14.7 and does so very quickly. Outside of this range it doesn't function which is why we have the TPS vs RPM fuel maps in our ECU's as well as the MAP vs RPM fuel maps.

When tuning you definitely want the stock O2 sensor disabled since it will throw some crazy AFR's at you trying to get the bike back to 14.7.