According to a recent Google+ post sent out by Googler Elliott Hughes, the 'Check for update' button now does more than it did before. To be clear, and as Hughes puts it – it now "actually works." Android device owners who prefer to be first in line to receive an OTA update will be well-versed in the ways of the Check for update button. Namely, the having to repeatedly tap the button when an update has been confirmed as on its way out to see if it has arrived yet.
Up until now that action was pretty much redundant due to the way in which updates previously worked and the use of quotas in place for each stage of the rollout. Where if that quota had originally been filled when the user hit the update button, it did nothing. Likewise the following (repeated) hits of the button after the initial button pressing also did nothing. This is where the change has now occurred though as Hughes points out that anytime the update button is now pressed, the system registers this as a user-initiated action and therefore makes the update available.
The major point here is even if the quota has been reached, as the action is being understood as a command by the user the update tool will make the update available. It is also worth pointing out that the posting by Hughes notes this is not the same for the standard background checks that are performed by the device – which still may show the update as unavailable after its last check. So even if the system is showing that it has recently checked for an update and that none are available, now tapping the check the update button will likely start the update process – at least that is the idea. As Hughes further points out this means that Android device owners now "shouldn't need to sideload an OTA or flash a system image just because you're impatient." To ensure the Check for update button now works as it is intended to Hughes notes that Android devices need to be running a current version of Google Play Services. Which is of course in addition to having a device that pulls its update directly through the default Android update tool, which likely means Pixel and Nexus devices.