If it actually does require a move/command to execute mega evolution, then there might be more Taunt users then there ever has been this gen.
I don't mean it being a move but rather a command that eats a turn but has priority over every other action except for switching Pokemon. Kind of like using an item on a Pokemon or the in-battle "Call" action in Pokemon Colosseum.
For a hypothetical situation, let's say that your Ampharos is stuck facing-down a Chlorophyll Venusaur or something on a rampage. Normally you'd be boned, but you call out to Ampharos to have it Mega Evovle before Venusaur attacks and now you resist all its attack. I feel like that's the kind of come-back situation they may be talking about. Of course Ampharos would be evolved for the rest of the battle and would block any other Pokemon on your team from Mega-Evolving even if they hold Mega Stones which would make a lot of sense with how they've explained the mechanics of Mega Evolution.
Mega Evolution really could add a new layer of depth if it works like I think it might.
For example: another situation would be Absol against a Deoxys-S. Deoxys could just smash Absol with a Superpower, expecting the Mega Transformation to give Absol Magic Bounce, but Absol could just as easily Sucker Punch and take it down instead. (I know they carry Focus Sash, it's hypothetical, cut me some slack.) The base Pokemon can do just as much work as its Mega Evolution can and it might even be best to save Mega Evolution for when you
need it.
One last example would be Mawile: Mawile switches in on, let's say a Gyarados. Mawile Intimidates it on the switch-in, then as Gyarados Dragon Dances to get back to +1 Attack or whatever, Mawile Mega Evolves with increased Defenses and Huge Power to now take charge with Draining Kiss and Sucker Punch coming from 590 total Attack.
The whole thing could be really deep in terms of strategy and honestly reminds me of the Pokemon anime. Ash's Pokemon has been doing work all battle but just when things get dire, the Pokemon evolves and comes out on top of a bad situation. The more that I think of the possibilities, the more excited I get for Mega Evolution as a whole.