BK is the abbreviation for knitting backwards (also known as backwards knitting).
Knitting backwards allows you to always work from the RS of your knitting, even though you are working flat. This technique requires that you throw the yarn. You may use either hand to throw. When backwards knitting or purling your right needle isn’t turned and moved to your left hand. The last stitch worked on the RS row worked right to left becomes the first stitch of the WS row, worked from the RS, from the left to the right.
Despite what you may think, this is not at all difficult to master. It is mostly retraining our hands which are so used to going in the right to left working.
Think of what you are trying to accomplish: Create a knit on the RS of the fabric, but working the opposite direction. Here’s how you make a knit:
To knit on the RS working right to left, hold your yarn behind the needles. Insert your right needle into the front of the stitch on your left needle, going in from left to right. Wrap your yarn over the top of the right needle. Move your right needle underneath your left needle, moving to the front. While guiding the new stitch onto your right needle, remove your left needle from the original stitch.
Knitting Backwards / Backwards Knitting From Left to Right
Setting up for knitting backwards / backwards knitting: Hold the yarn in either the right or left hand as you prefer. The needle with the stitches will be in your right hand.
1) With your yarn in back, put the left needle into the back leg of the stitch on the right needle so the left needle is in back.
2) Wrap your yarn around the left needle, wrapping your yarn from behind the left needle towards the right needle, going over the top of the left needle, going under your left needle to the starting position.
3) Bring the left needle underneath the right needle (or the right needle over the left), moving the left needle towards the front, and out of the stitch on the right needle;
4) As your new stitch is formed on your left needle, remove the stitch from your right needle to the left needle. Your new stitch should look like a knit stitch with the yarn coming from the front leg of the stitch just created.