UA's next step in turning the passengers upside down and shaking every penny from their pockets is to sell reclining seats at a premium (throughout the aircraft). You will not be able to sit there unless you paid the premium.
Perhaps there will be a special card you are given to slide into a slot to unlock the reclining feature. The card is only good for 1/2 that segment.
(like the key cards one gets at hotels). A warning flashing light and beep will go off 15 minutes before your time is up at which point the seat will spring back quickly to an upright position, regardless of whether you are awake. So be sure to keep your seat belt fastened when using this feature.
You can update the card for the second half of the segment by ringing the call button for the FA who will stop serving drinks to come swipe your credit card.
Also, UA will sell recline-free seats at a premium. The seat in front of a person will not be able to recline.
However, for a discounted price, one can buy a year-long subscription for either feature.