Tiny's Trial
Tiny's Trial is a reward minigame sometimes found in Betrayal league Safehouses when Guff is one of the members of that division. It allows the player to repeatedly enhance an item for free, under time pressure and with a limited set of options.
Preparation
To start the minigame, the player must select a normal, non-corrupted item, then press a "Start Timer" button. Before starting the timer, the player can use a portal to return to the stash to retrieve a desired item, as long as there are portals remaining to return to the Safehouse.
Gameplay
Once the player inserts an item and presses the button, the item is locked in place, and the timer starts. A selection of currencies appears, often including high-level currencies such as 
Chaos OrbsChaos OrbStack Size: 20Reforges a rare item with new random modifiersRight click this item then left click a rare item to apply it.
and 
Exalted OrbsExalted OrbStack Size: 20Augments a rare item with a new random modifierRight click this item then left click a rare item to apply it. Rare items can have up to six random modifiers.
. These currencies are free (they are not taken from the player's inventory or stash). The player can apply these currencies to the selected item until the timer expires, at which point the unused currencies disappear and the item can be retrieved.
The gameplay can be simulated at https://poesimulator.github.io/trial/. In this simulation, players left-click to apply currencies, rather than picking items up with the left mouse button and applying them with the right, as they do in the real Tiny's Trial.
Strategy
The key to this game is to rapidly choose a good sequence of item enhancements from the available options. For example, only a single [Essence] can generally be used (since an Essence can only modify a normal item). So if multiple Essences are available, the player must rapidly choose one. Orbs of Scouring can be very useful if available, since they allow the player to start from scratch if a modification goes wrong. Vaal Orbs are very dangerous, since they corrupt the item and thus lock out most if not all of the other currencies.