In knitting, SSK, or slip-slip-knit, is a single decrease, not a double decrease. A double decrease can use SSK but must also contain another action to complete the second decrease. Some forms of double decreases do not use SSK at all.
On its own, an SSK decrease slants to the left on a knit piece. To accomplish this from the knit side, a knitter first slips one stitch, then another, from the left needle to the right as if to knit, without knitting the stitches. Next, the knitter inserts the left needle, from left to right, into the front loops of both of the slipped stitches. Then, the knitter completes the decrease by wrapping the yarn around the right needle and pulling it through the loops as if the two slipped stitches were one regular stitch.
A double decrease containing SSK slants to the right on a knit piece. To make a double decrease on the knit side of the work, a knitter performs the SSK, then slips that decreased stitch back to the left needle without twisting it. Next, the knitter uses the right needle to lift the second stitch off of the left needle by taking it up and over the decreased stitch. Finally, the knitter moves the decreased stitch back to the right needle, without twisting the stitch.