incL is the abbreviation used for knitting into the stitch below in a way that makes your increase lean to the right. Then the incLp is a variation where you purl instead of knit. The mirror stitches of these are the incR and the incRp.
incL – increase left
incLp – increase left purl
These same increases are used and called other things by other authors. I decided to use this method of abbreviating my increases because it clearly states it is an increase, that it leans to the left, and then if needed, that the stitch made should be a purl.
How to incL
This increase must be made in a stitch that has already been made, so it is done after a stitch is made (that is, it cannot be done until at least one stitch has been worked in a row). It can be worked into a last stitch, but I don’t usually recommend that.
Use the left needle to pick up the top of the stitch below the stitch just worked. Lift stitch onto the left needle and knit it. Count the increase.
How to incLp
Use the left needle to pick up the top of the stitch below the stitch just worked. Lift stitch onto the left needle and purl it. Count the increase.
I usually work both of these increases without lifting the top of the stitch onto my left needle. This takes a little practice, but feels more efficient, and once perfected, puts less strain on the stitch while working the increase.
How to work an inc2p (also inc3)
Purl into top of stitch below the first stitch on left needle (see incRp) then knit the first stitch on left needle and then purl into the top of the stitch in the row below the stitch from the left needle just knitted (see incL, above). This increases 2 stitches from one stitch. Count 3 stitches.
Historical note: Patterns published before 2011 by Y2Knit have the incL counting as 1 stitch, but the incR was counted as 2 stitches because I included the increase and the stitch worked after it from the left needle as a single instruction. The difference from how I counted the incL caused a lot of questions (and consternation), so I discontinued counting the two decreases differently. When a pattern is updated (patterns downloadable to date have been changed), I change to the current method of only counting the increase and make all resulting changes.
See also: incR