I updated the link to point to the docs, which are more informative. As far as I understand, flambda is an intermediate representation of OCaml which allows for a number of optimisations and better inlining. You can check if you already have flambda enabled by runnning "ocamlopt -config | grep flambda".

If you're using opam then you can test flambda out by creating a new switch

  opam switch create ocaml-flambda ocaml-variants.4.14.1+options ocaml-option-flambda

and then running "opam switch set ocaml-flambda". (You can replace 4.14.1 with your preferred version above.)

It's a bunch of optimisation passes, I believe mostly around inlining. Ocaml is historically quite passive on that.