Split-Screen Reaction

The villain of the day has dropped a bombshell revelation. To show the reaction shots of all the protagonists at once, the screen is split into sections, with a reaction shot in each section. The reactions may show up one at a time and accompanied by a clang sound effect.

Alternatively, the split screen may show the faces of all of the protagonists as they act in unison. This is especially popular in shows with Combining Mecha, Colors or Powers.

A typical Split Screen is just two people and a thin vertical divide between the two halves. The Split Screen Reaction often has stylized divisions at strange angles. It may not even be a true "split", but a series of picture-in-picture bubbles that pop up. (Also see Odd-Shaped Panel and Hexagonal Speech Balloon.)

Characters with Medium Awareness may be able to use the split to interact with the other characters where it wouldn't usually be possible.

When parodied, it's usually part of an overdose of Anime Effects, resulting in What Do You Mean It's Not Awesome?

Compare Multi-Part Picture.

Examples of Split-Screen Reaction include:

General

Used a lot in Mons shows like Pokémon, where an offscreen character will yell commands or encouragement to the onscreen characters as they act.
Just about every Shonen series does this when a particularly harsh blow lands.
To be fair, most anime were there is any sort of competition will show something like this.

Anime & Manga

Bleach tends to use this effect a lot. The first few frames of this epic spoof are an example.
Ditto for Naruto; well at least during the Chuunin arc.
Happens in the first episode of The Law of Ueki, when Ueki's tree appears from the river.
Happens occasionally in Outlaw Star.
Happens several times an episode in Katekyo Hitman Reborn.
Seen in the Akira manga. Plenty of characters, plenty of shocking events to make them react.
In Corsair this happens over dinner when Ayace suddenly announces his relationship with Canale. Everyone at the table gets a reaction panel, with expressions ranging from awkwardness to surprise to confusion.
The Fairy Tail anime has split-screen constantly, with a lightning bolt-like separation. Most of the time the screen is split just in two, though there are a few multiple splits too.
Given that Saint Beast has six protagonists and gets a lot of mileage out of Shonen tropes, it was destined to happen.
Similarly, One Piece has several protagonists with a strong tendency to overreact (with the exception of Robin), so this trope is a given.

Films -- Live-Action

Honorable mention to the Ominous Multiple Screens at the end of The Matrix Reloaded, which display reaction shots of all the previous Chosen Ones, or is it the different possible reactions of Neo himself, but in the end, it doesn't matter since they're all portrayed by Keanu Reeves.
Happens with Ramona, Knives, Kim, Stills and Young Neil in Scott Pilgrim vs. the World after Gideon stabs Scott with the Power of Love.

Video Games

Pokémon uses this quite a bit.
The first Super Smash Bros. Brawl trailer had all the present smashers react in shock this way to seeing Wario about to do a Wario Waft.
Used in both Persona 3 and Persona 4 as the start to an "All out attack" when all surviving party members gang-rush downed enemies in battle.

Web Comics

Used in Juathuur, for example here.
Parodied in this strip of The Way of the Metagamer.
Seen in The KA Mics when Zog gained webcomic awareness during the Crossover Wars.
Done in Dubious Company twice.
- Done seriously in the showdown at Kreedor Castle when Izor triggers the sacrificial ritual on the racoon instead of Sal.
- Again in the Recycled in Space arc when the Marines pass judgement on Walter and his ship's passengers. A hilarious variety of reactions ensue from Walter, Sal, Tiren and Marty, Gary, Mary, Sue and Izor, Elly, and Leeroy.

Western Animation

Can be seen in Wakfu, as with the above picture. Not surprising given the show's Animesque roots. Especially common during the Bontarian Gobbowl arc.
Quite common in The Amazing Spiez, which regularly uses comics-style separations for the screen. Alongside the regular, rectangular panels, you'll have the sharp angles typical of this trope once or twice an episode.
Also a common sight in Monster Buster Club. Including one in the Action-Hogging Opening.