OK I did not explain very well-sorry! I'll try to now.
If all of the sheep/goats in a flock are dewormed at the same time then only "strong" or resistant worms are left. If a good portion of the flock is Not dewormed then the animals that were not dewormed will have "weak" or susceptible worms. The idea is that the worms will cross breed with each other reducing the number of "strong" worms. Therefor the drugs should continue to work according to this plan.
For more info you can go to http://www.scsrpc.org/ This web site also talks about alternative means of deworming such as copper particles and certain plants that have high levels of tannins like bird's foot trefoil. It also gives tips on how to reduce the load of worms on you pasture. For ex: if when you deworm you keep the sheep off pasture in a pen (you can feed hay) or in the barn then the manure with a high load of worms is easy to deal with (you can compost it.) This web site is loaded with information!!