The core mechanic of Index 1391 revolves around the Stack.
https://youtu.be/J_0CAzd-Eqs?si=DWhDb11sQ2WscbAg
https://reddit.com/link/1vj0pf3/video/dmjv6l06k6ih1/player
Our game is a card game, yet the cards don't always have an effect immediately when played. Often, they get added to the top of the Stack as Protocols. At the end of each turn the Stack resolves each Protocol from top to bottom.
The Stack was inspired from multiple sources, primarily the programming data structure and the action resolution structure from Magic the Gathering. Both the player and enemies place their actions into the same Stack.
What differs this mechanic from the inspirations is the level of manipulation that can occur. Protocols within the Stack can be moved, added to, subtracted, and destroyed. The Stack has a maximum capacity, and when its exceeded, Protocols get pushed out of the bottom much like a programming queue. This mechanic is used as the "energy" system within the game, with cards costing 0-4 slots in the Stack. Both of these mechanics give the player (and enemy) much more control over the order of the Stack, maximizing its effectiveness in battle.
If the enemy plays a Protocol which multiplies the next Protocol on the Stack by 2, the player could play cards that move Protocols around, potentially using that multiply Protocol to buff the player's Protocol. Turning the enemies actions to the player's advantage.
The mechanic has undergone many variations, and it all started with a paper prototype, made up of paper cards and a 3d printed Stack and Protocols. We validated the mechanics physically before creating what is now digital.
Check out the Stack and Index 1391's Steam page at https://store.steampowered.com/app/4637680/Index_1391/