There are lots of informations in "Concurrent Programming with Effect Handlers" (https://github.com/ocamllabs/ocaml-effects-tutorial) on what it looks like and how it's implemented. There are also more recent informations on the September 2021 edition of the Multicore OCaml newsletter: https://discuss.ocaml.org/t/multicore-ocaml-september-2021-e..., with a link to a paper about effects in OCaml.