Reaching a PSE Burst
PSEs (Photon Sensitive Effect) are boosts that appear in the top right of the Screen. The higher the level, the better the boost will become. PSEs will level up when enemies are defeated and a pillar of condensed photons shoot out of the enemy.
As a side note: A lot of information has spread about Reaching PSEs and what not, but it is really just defeating enemies and hoping for the best.
Reaching a Level 8 on a PSE will give you a chance at achieving a PSE Burst. Enemies will start spawning in the location of the Players. Once an enemy is defeated and the corresponding PSE appears again a PSE Burst will occur.
PSE Burst!
Once you've PSE Burst, all the players will want to stick together. Enemies will now rapidly spawn in your location. You'll still be able to generate PSEs while in a Burst. There is also a chance of receive a ONE MORE and extending your Burst.
Cross Bursts can be achieved by reaching more than one Level 8 PSE. Once a Cross Burst is reached, there will be a guaranteed ONE MORE.
There should be a clarification that aside from spawning enemies rapidly, the level 8 PSE effect is all it is. There's a LOT of confusion about this.
Also should clarify that it has nothing to do with emergency trials spawning bosses. Some PSEs can increase the rate of rare bosses in trials but some people even think that PSE Burst in general has to do with spawning ragnes.
Good things to add:
1) If a ONE MORE (01+ Burst) has been achieved PRIOR to getting a Cross Burst, another ONE MORE (02+ Burst) is also guaranteed.
2) Spawns in a PSE Burst are dictated strictly by each player in the quest having a "spawn radius" around them, meaning mobs will only spawn near players.
3) Each player has 1 mob slot, or 1 unique PSE mob that can spawn for them at a time. This means that if all players rush to group up and someone doesn't kill their PSE Burst-specific mob, no more mobs will spawn from that character's spawn radius because their mob slot is already taken elsewhere on the map.