FSRS4Anki is an Anki custom scheduling implementing the Free Spaced Repetition Scheduler. FSRS4Anki consists of two main parts: scheduler and optimizer.
The scheduler is based on a variant of the DSR (Difficulty, Stability, Retrievability) model, which is used to predict memory states. The scheduler aims to achieve the requested retention for each card and each review.
The optimizer applies Maximum Likelihood Estimation and Backpropagation Through Time to estimate the stability of memory and learn the laws of memory from time-series review logs.
For more detail on the mechanism of the FSRS algorithm, please see this paper: A Stochastic Shortest Path Algorithm for Optimizing Spaced Repetition Scheduling.
For the tutorial on FSRS4Anki scheduler, optimizer, and simulator, please see: Usage
Here collect some questions from issues, forums, and others: FAQ
Some add-ons intervene in the scheduling of Anki, which would cause conflict with FSRS4Anki scheduler. Please see Compatibility for more details. I will test these add-ons. Let me know via issues if I miss any add-ons.