It's recommended that it's used before fermentation for the most cost effective usage. Depending on the method you're using, it might be better to add it a couple of days or so before pitching the yeast (oh, and I believe it's killed off/retarded if you apply heating to a must that has had it added).
Equally, it seems that it can be used in batches that won't clear too, but the dosage suggested is twice to three times higher than if it's used before fermentation.
There is some anecdotal evidence that suggests it not only helps with pectins, but also colour and flavour extraction too, so use before fermentation would be helpful for that.......