4d6 On Fire: At the start of your turn, remove a die from the stack and roll it to see how much damage you take from the flames.
3d12 Trapped by Rocks: You can't Move until all the dice have been removed from this stack.
1d10 Berserker's Rage: As long as this stack is around, add another die to the stack whenever you take damage.
There are no hard and fast rules on making special stacks, but most stacks probably shouldn't be special. Not only would making lots of stacks special mean lots of rules that everybody needs to remember, special stacks used in moderation give flair to specialized items (such as magical equipment, incendiary devices, or poisons) and powerful character special abilities that wouldn't feel as awesome if being a special stack were more common.
Flashback stacks are a special stack built into the game that are created when you use the Prepare ability. In addition to using the dice on any roll that it makes sense some extra preparation beforehand could have benefited, flashback dice have the special ability to allow you to step back in time.
When your GM describes a complication that your preparation could have resolved, you can spend one of your flashback dice to take the narrative back in time to describe how your preparation dealt with that problem.
An adventuring party in Victorian London is investigating an elusive murderer. In an effort to capture them, the group takes a few days to take in the crime scene and Prepare.
Later, the group finds themselves deep in the sewers chasing after their quarry. In an effort to catch up to them, the mastermind of the group flashes back to purchasing a sewer map from a descendant of one of the original architects. The player marks off the cost of the map and uses it to corner the suspect and force a confrontation.
Not every preparation is guaranteed to succeed, however. Your GM is well within their rights to ask you to test an attribute to see how well your preparation holds up, or even if things go well for you. If they do, you can use any number of flashback dice on the roll -- including any you haven't spent yet!
An adventuring party plans to steal from a high security vault. They take a few days to case the joint.
Later, the group is deep within the compound when they discover an impenetrable web of security lasers. The hacker flashes back to making preparations to temporarily disable the security. The GM asks them to roll to see if they're able to bypass the vault's security-- and for how long. The hacker manages a partial success-- long enough to get them in to grab what they needed, but not long enough to keep them from tripping the lasers on their way back out the door!
No rewriting history
It's important to note that flashback dice cannot rewrite history. If a guard sounded the alarm, no amount of flashing back to bribing them will undo that the alarm has been pressed. However, if the GM describes the alarm being pressed but hasn't yet described the blaring alarm, it's totally valid for someone with tech savviness to cut in with a flashback to disabling the alarm system before the party made their way in.
That said, don't be a jerk about this rule. The goal is to encourage a flow to the improvisational "yes, and..." of playing a roleplaying game, not to make every dramatic encounter a race to using a flashback die before the GM finishes describing a new threat.
GMs, give spaces for players to offer up flashbacks and prompt your players if you think they may have forgotten.
Players, trust that your GM will offer you these opportunities and don't interrupt them in an unwelcome fashion.