This cable is worked across 4 knit stitches, crossing 2 stitches with 2 stitches.
To create a cable that slopes to the left you slip the first 2 stitches onto a cable needle and hold that needle at the front of your work. Now you knit the next 2 stitches. Finally you come back to the 2 stitches on your cable needle and you knit them.
Here is a video of how this is worked:
This is know as a 2/2LC (a 2 by 2 left leaning cable).
If you see it in a chart it will look like this: