This was written up in a kind of difficult way to read, but the basic takeaway for me is that players who are ubered while not actively being beamed drain the uber at a faster rate. That's why when you beam them, overlapping your "free" uber with the "flash" uber, the drain goes back to normal.
Some gameplay applications:
1. Breaking beam during a solo uber proportionally as bad as flashing, so don't do it lol. Or account for it if u do and leave earlier.
2. The "free" flashes at the end of an uber aren't technically actually free, it's just that the tradeoff (1 second flash for 0.5 off the original uber) goes from a 2x ratio to approaching an infinite value as the uber approaches its end. (flashing x people with y < 1 second of uber remaining will shorten it to y/(1 + 0.5x) while generating x total seconds of flash. Let's say we have 0.25 seconds of uber left and a robotic medic who flashes 4 people in that span. That shortens the original uber to 0.08 seconds and creates 4(!) seconds worth of uber from thin air. Your net uber gain is 4/0.16 = 24x return!
That said, gaining real value from it is easier said than done. It won't matter in even uber trades since both medics have access to this (unless one medic diffs the other? OMG), and if your uber is clearly better to the degree that you can take advantage of this by having your team flood at the last second, it's likely the other team will already know this and back off/scatter enough to where it doesn't really matter. Teams pretty much already run away to reset towards the end of uber trades regardless.
I think the most realistic way I can see this getting value is an aggressive yard to second trade on granary where you flash your entire team in at the last second and count on trading out favorably or forcing the enemy out of the point. Even then, there are complications since the force has to be near instantaneous in order to gain value from forcing flashes.
Also theres no point when u can just run a sniper or something and be way more successful with no risk LOL
edit: wait it might be goated on koth tho