Introduced in 2003, the product costs $17.95 per pair and can only be used while using tray tables that unfold from seat backs. The gadget slots on to one of the struts that hold the table in place jamming the seat in front in an upright position.
There are reports that the gadget has caused arguments and broken seats but GadgetDuck.com states on their website "What the FAA says" from a 2003 Washington Post report: the clips were not against federal aviation rules as long as they weren't used during taxiing, takeoffs or landings.
And so the argument continues on when it is right to recline an airline seat. Internet reports indicate the device does work preventing recline of no more than one inch. Obviously the passenger whose seat is obstructed to recline will know something is up so being discreet about using the device is out of the question. Expect flight attendants to step in and ask the gadget to be removed.
The most polite passengers will let the passenger behind them know they intend to recline. Myself I usually don't recline if I know the passenger behind me is using their tray table to eat a meal. Best advice: some seats have no recline due to their proximity to emergency exits; book the seats behind them.