It makes sense!
If I've understood properly, you're knitting something like 10 stitches of red, followed by 5 of black, then 20 of red, yes?
So for this, you have two balls of red, and a small one of black. Knit your red, start the black by leaving a long end, then work the black, then start the second red. For your next row, do the red, then before starting the black, twist it once around the red wool (like crossing it at the back of your work) then work the black. Do the same for the red at the end. You just need to twist the colours once so you don't get a hole iin between the different sections. If you carry the red across the scarf behind the black, you risk holding it too tightly and bunching up the scarf.
For a proper explanation, google "intarsia"
If you were doing something in checks eg 2red 2 black right across the work, you would carry the out of use colour behind the colour you're using. For short areas, there's less risk of bunching, but you will generally end up with a slightly tighter finished piece.
Hope that helps.