Current viewport heuristics for speculation rules don't give any room for developer experimentation. This experimental feature will provide such controls, and enable developers to figure out if different heuristics parameters give them better results than the default ones. This is a feature only aimed at experimentation, and there are no plans to ship it as is.
Explainers: https://gist.github.com/yoavweiss/6bb2de06642b475a12205684aea2eef9