It needs to be done after the film is exposed. It is not the same a pre-exposure. It needs to have a minmum of 15 minutes and requires really dark conditions. It will only work with film that has been exposed to normal conditions...it will not work with film that required reciprocity compensation for exposure correction, I know of no reason that low light latensification can not be used on a frame that was pre-exposed but one would have to be quite careful not to get a very low contrast result. I did not get spurred into trying this idea from David Vestal but a from a Vestal edited article due to work and writing by Ralph Steiner and that David did a wonderful job of editing, Although I had read about it at least 30 years earlier I finally get around to trying very recently.
You need to do your own work to set it up. The light level being used is so dim that one can hardly see it much less measure it and specify a set up.