It has zero effect at all -
its a waste of money, I tried it and FEC before and after, it made zero difference at all, and My vet confirmed that in every case where people used it and forwent worming or good pasture management, it resulted in increased worm burdens, or at best no change.
It is nothing more than hopeful thinking and snake oil -
The best thing to do is manage your pasture rotation properly, and this will allow you to keep worm counts low, I myself have not wormed any sheep this year, and only 20 of 340 last year, with my FEC readings rarely topping 120. This is by ensuring sheep never stay on grass for more than 10 days at a time before being moved, and a minimum of 60 days rotation time in summer (during the egg hatching season), ideally pushing it out to 100 days where possible.
Also keeping grass covers above 4-5cm, and ideally 10cm on entry. impacts on the larvae survival as fewer reach the top of the grass.
Simply you would be better saving your money and spending it on other things.
As for putting it on hay - hay is an inert feed and sheep fed on forage will have lower worm and parasite burdens simply because the food they are eating has no parasite eggs on!