The bar increase is a visible increase in Stockinette stitch and tightens the stitch in which it is worked. Because it is visible, it is often used as a decorative stitch when working in Stockinette stitch. To perform the bar increase, you will either knit into the front and back of a knit stitch or purl into the front and back of a purl stitch.
The bar increase works very well when used within ribbing because it can be placed in the transition between a knit and a purl stitch. On the right side it creates what looks like a knit stitch followed by a purl stitch. For example, when asked to increase evenly within a k1, p1 ribbing, use this technique to increase into the knit stitch (k f&b). The pattern will then become k1, p2 after increasing.
Knit into the front and back of a knit stitch (k f&b)
Purl into the front and back of a purl stitch (p f&b)