What is an event-ready scheme (at-a-glance) in the context of Bell-type tests? The abstract of Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometres says

We use an event-ready scheme that enables the generation of robust entanglement between distant electron spins (estimated state fidelity of 0.92 ± 0.03).

I'm not asking for a complete unpacking of the subject. What I would like to know about event-ready schemes is the gist of what they are, and how they achieve the relatively high state fidelity.
 A: The paper cites (Żukowski et al. 1993) when mentioning "event-ready schemes". In this paper, the authors write:

This technique, which we call entanglement swapping, leads to a realizable scheme of "event-ready detectors." In such a scheme, which has
been called for by Bell since 1971 [2], one knows, via
some initiating event, when a pair has been produced.
Consequently, one can measure directly the probabilities
of the various outcomes, including even nondetection of
the particles

From this, I gather that they are referring to what is also often referred to as "heralding" nowadays. The relevant context is that in quantum optics, when an experiment requires using single photons, the photon sources are generally non-deterministic. This means one cannot actually be certain when photons have been generated, and therefore cannot be certain whether, say, not observing photons at the detectors was due to the photons not having been generated at that time, or is due to something else (say, photon losses along the apparatus).
Remember you cannot just measure whether a photon was generated before sending it through the optical circuit, as measuring the photon destroys it.
To obviate this issue, one can use a scheme where there is a certainty that a photon was generated and sent through the relevant quantum optical circuit. For example, if using an SPDC source, photons are generated in pairs, and so one can measure whether a photon was generate on one side, and in that case be certain that another photon was generated somewhere else (there's specific angles at which photons are generated etc).
This kind of scheme seems to be what the authors are referring to here.
