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Forcing a game's 'random' number generator to spit out exactly the results you want.
Most games use pseudorandom number generators: algorithms that produce seemingly random sequences from an initial seed. If you know the seed and the algorithm, the output is entirely predictable. Speedrunners exploit this by performing specific actions at specific times to advance the RNG state to a known position, guaranteeing favorable outcomes like critical hits, rare drops, or enemy behavior patterns. Some runners count frames or use audio cues to track where the RNG state is in real time.
Example
In Pokemon Red/Blue, runners manipulate RNG by performing precise inputs on specific frames to guarantee encountering the right Pokemon with the right stats. The entire run is essentially a choreographed dance with the random number generator.
Why it matters
RNG manipulation removes luck from speedruns, turning them into pure execution challenges. For developers, it's a lesson that pseudorandom isn't truly random, and if your game's fairness depends on randomness, determined players will reverse-engineer it.
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