When you are knitting a rib, either 1x1, 2x1, 2x2... and you want to change to single jersey, all the stitches from the rear bed need to be transferred to one needle bed (front or rear).
Watch this video to learn how to insert the transfer rows:
You need to select a cell at the top of the rib and right-click on it. Go to "Insert rows" and enter the desired number of clear rows you need to add. Then use the pencil tool to draw the transfer stitches (rear to front).
Remember there's a limitation for transferring adjacent needles so you can only transfer every other needle in one pass.
1x1 to jersey
2x2 to jersey
2x1 to jersey