Tapping the “Check for Update” button on Nexus/Pixel will actually be useful now
Since only 1% of devices are usually selected to get a new Android update in the first 24-48 hours, it's quite easy to miss the opportunity. Subsequent taps on the button won't do anything until the next batch of devices is scheduled to start receiving the update.
The good news is things have changed, as Googler Elliott Hughes points out in a post on Google+. According to Hughes, as long as you have the latest version of Google Play Services installed on your phone, you can bypass the 1% limitation when you tap the “Check for Update” button.
If you’re running a current Google Play Services, you shouldn’t need to sideload an OTA or flash a system image just because you’re impatient. How does this interact with the gradual rollouts of new releases, you ask? When a device checks in because you’ve specifically asked it to, we flag that this is user-initiated and so you’re not subject to the usual limitations. So even if we’re at 1% rollout and 1% of users already have the update, if you manually check you’ll still be offered it, even though a background check at the same time wouldn’t.
source: Google+ via AndroidPolice
Things that are NOT allowed: