The short pre- or post-exposures that I'm familiar with are at an intensity above that at which Low Intensity Reciprocity Failure (LIRF) occurs. The post-exposure latensification mentioned by Claire occurs well into the LIRF region - so far in that very little overall fogging occurs. The extremely low intensity light is capable of amplifying the image, much like Becquerel discovered all those years ago. Latent sub-image centres (Ag2, stable, undevelopable) can become latent image centres (Ag4, stable, developable). The light intensity is such that most non-image grains never become developable - they only ever get to be non-latent image centres (Ag1, unstable, undevelopable) that recombine in an instant.
I'm using 'Ag2' etc to mean two silver atoms together.
It is like the reverse of hypersensitisation - in which all grains become latent sub-image centres, so that it only requires one good photon to push the sub-image centre to an image centre.
Hope that makes sense.