Swatches 4-6 demonstrate three different sorts of increases. They each look different, and they each are varying degrees of “invisible.”
The bar increase, my favorite when it doesn’t matter, leaves a horizontal bar across the bottom of the stitch. If you look at the toe of any sock I’ve ever knit, you’ll see them.
The M1 (make one) increase is a little more invisible, although you can see it if you’re looking for increases. This one can slant left, slant right, or leave a yarnover-esque hole (useful when doing openwork or lace). The trick to keeping the hole closed is to knit into the back of the stitch, twisting it and closing up the hole- the slant comes from whether you put your left needle into the stitch from the front or back, and I forget which is which. I always have to look it up.
The lifted increase is very difficult to explain, but is sort of a cross between the two. You pick up a leg of the stitch in the row below. It’s the most invisible of the three, but you need to remember to work at the very tippy tips of your sharpest needles for them, or else they stretch.
From the bottom: bar increase, M1 increase, lifted increase (swatches 4, 5, and 6).