pageHideTimeout: ... a timeout in milliseconds after the next pagehide event is sent, after which a beacon will be queued for sending, regardless of whether or not the page has been discarded yet.

The main intention for this property is to shorten the wait time after a page goes into bfcache and frozen. A page could send a beacon earlier before it's actually unloaded (or evicted from bfcache).

Some offline discussions suggest to not limit it to only pagehide, but also include visibilitychange or freeze, mainly for easier doing analytics. For now, users can approximate it by executing setTimeout(sendNow(), timeout) in the corresponding event listeners.