The k2tog tbl and the p2tog tbl are decrease techniques very much like the k2tog and p2tog decreases - except that you knit them through the back of the loops i.e. tbl.

( Note: 'loop' = stitch )

What this means is that instead of knitting or purling through the front of a stitch on the left-hand needle, you instead take your right-hand needle through the back of the stitch, behind the left-hand needle.

For a knit stitch, 'tbl' means you would take the right-hand needle down through the back of the stitch, and for a purl stitch, 'tbl' means that you would take the needle up through the back of the stitch.

k2tog tbl = Knit two together through the back of the loops.

This is a single decrease, so it reduces the number of stitches on your needles by 1.

It's used mainly on knit rows, and in appearance it resembles the ssk decrease.

p2tog tbl = Purl two together through the back of the loops.

Again, this is a single decrease, and is the purl equivalent of the k2tog tbl.

It's used mainly on purl rows, and resembles the ssp decrease.

Both the k2tog tbl and the p2tog tbl are left-leaning decreases, so when looking at the front of the knitting, they will appear to slightly lean to the left.

You might see these decreases pop up in lace knitting patterns.

Here is the tutorial: