BP is the abbreviation for purling backwards (also known as backwards purling).
Purling 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 when working right to left becomes the first stitch of the WS row, worked from the RS, from the left to the right.
This is slightly more difficult to do than knitting backwards, just as purling is more 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 purl bump on the RS of the fabric, but working the opposite direction. Here’s how you make a purl:
To purl on the RS working right to left, with the yarn in front of the needles. Your right needle is inserted into the front of the first stitch on your left needle, going in from right to left. Wrap your yarn over the top of your right needle and underneath. Your right needle is moved underneath your left needle, guiding the new stitch onto the right needle. Now remove your left needle from the original stitch.
Purling Backwards / Backwards Purling From Left to Right
Setting up for purling backwards / backwards purling: 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 the yarn in front, put the left needle into the front leg of the stitch on the right needle so the left needle is in front.
2) Wrap the yarn around the left needle, wrapping yarn from in front of the left needle towards the right needle and under (back) the left needle and between the two needles. Pull the yarn towards the left needle or backwards to create tension.
3) Bring the left needle underneath the right needle (or the right needle over the left). Move the left needle towards the back of the right needle, and out of the stitch on the right needle.
4) As your new stitch is formed on your left needle, remove the original stitch from your right needle. Your new stitch should look like a purl stitch with your yarn coming from the front leg of the stitch just created.