Sounds tricky, but believe me it isn't. I am hoping that showing a cardigan worked in stripes doesn't make it look harder than it is, either.
The shaping is worked over five stitches and on every right side row, that is, with the front side of the work facing you. The method for knitting right side rows in fisherman's rib is shown on the last three photos here.
The central stitch of the five is worked as a purl on the right side and knit on the wrong side. If it is slipped and the yarn passed over the needle, then the decreasing stitches end up too loose and messy.
The central stitch marks the underarm, the point where the pieces are joined onto a circular needle, as can be seen here.
One every right side row, the five stitches are worked as follows: knit 2 together, purl 1, slip 1 stitch knitwise, knit 1, pass slipped stich over.
After the first row of decreasing you have two knit stiches on either side of the central stitch. On the next wrong side row the five stitches are worked as purl 2, knit 1, purl 2.
On the second and every alternate right side row a slipped purl stitch comes immediately before and after the 5 stitch sequence. On the right hand needle in this photo you can see that the yarn has been passed over the needle over the slipped stitch, and 2 stitches have been knitted together.
Next, the central stitch is purled.
The next stitch is slipped knitwise.
The next stitch is knitted and the slipped stitch is passed over it (exactly the same as when casting off stitches).
The rest of the row is worked according to the method for fisherman's rib, so the yarn is brought forward and the next stitch slipped purlwise.
The next wrong side row will have no interruption to the knit 1, purl 1 sequence when you reach the five stitch sequence.
I have experimented with different methods of decreasing for fisherman's rib, and this is the way that was easiest to work and gave the neatest finish.