I have a big problem with slugs in my flower gardens as I use a lot of shredded mulch and pine bark nuggets. I have found that in the early morning after a rain or when there is dew on the grass is the best time to find them. The slugs come out of my gardens onto the grass , I am guessing to graze. I don't like them, so I cut them in half with a pair of scissors and they die. Not very humane you might say, but I am not into picking up slimy creatures.
Sluggo pellets also work good, but can be quite costly with as many large gardens as I have. I would be interested to hear how other gardeners deal with slugs.
This is how I understand stacking. First in an area you grow a quick maturing vegetable like radishes. As they are maturing, you can plant another vegetable near them that will grow and mature once the radishes are gone. There was one illustration of two rows of green beans with either melons on cucumbers growing in between the beans. Theoretically, when the beans are done, the cantelope/cucumbers vines will be growing to take over the bean row spaces. The goal is to grow as many crops in a given space, not wasting a lot of area between rows or sections of plants.