Releasing the grey clip (lock tab) is step one. Once that's slid back (it only moves a little, maybe 2mm), you can release the tab that actually keeps the two halves of the connector locked together. I inserted a small flathead screwdriver to lift the black tab up, but this isn't always necessary. YMMV. The connectors for the secondary O2 sensors (under the car) work the same way. Hope this video helps someone out... I struggled with these BMW O2 sensor connectors for a while before figuring out how they work.
