DansDeals is running a story (one that has been around before though) that gives you access to Business Class Saver award space even if there is none on a United flight.
The post isn't easy to understand for first-timers but here's the gist – using the 'Plan B' you are betting on Business Class Saver award space opening up before departure on a United flight. This is known to happen on some routes e.g. SFO-KIX where many seats are released just days/weeks before departure.
Here's what you need to do:
1) Find coach saver award space when logged in. If you have the United MileagePlus Visa Explorer you will see more award space opening up. Regular United MileagePlus members (and Star Alliance partners) will only see X inventory while credit card holders and elite members will see XN space as well.
2) Call the MileagePlus Web Support Team to make the booking.
Instruct the agent to make a booking in economy but wait-list you in business class. Although United does not easily offer such, a booking agent can seemingly make it happen. This may require finding the right agent and lots of calls.
You will be debited the business class award price and get an economy seat, though!
3) Now keep checking for award space (you may get a notification before that) and if not, automatically confirm any upgrade to business class.
If it clears you are all set.
4) If it does not clear 24 hours before departure you need to try your luck with an airport agent. Now I haven't done that before but the trick is to get a PR-1 status for your booking.
5) Should your upgrade not clear before your flight you can call the same MileagePlus Web Support Team to get your miles refunded!
That's a lot of effort for the rather mediocre United Business Class. But still, if there is a direct flight and you are constrained in your stop-over options (e.g. because you fly your in-laws around) then this is a good tip!