It's the exact same thing as abusing for egg IVs. NPCs advance your frame by 1, etc. The only difference is the amount of frame advancements you do. For Sweet Scent, it advances the frame by 2, as opposed to the egg advancing it by 1.
The encounter slots are just what Pokémon you'll encounter when you use Sweet Scent, fish, etc. If you right click on the spread you want, click "calculate encounter slots", it'll give you the encounter slot for that spread, as well as links to see what Pokémon appears on that encounter slot for each route.