Normally, half of your stitches go on the instep and half on the sole. So if you cast on 70 stitches, that would be 35 each. But if you had a pattern for the instep that was a repeat of 4, for example, you'd want 36 stitches on the needle(s) holding the instep and 34 on the needle(s) holding the sole.
Let's suppose that you need to take 4 stitches away from the sole to add to the instep. You don't want to take any four stitches, you want to add 2 to each side of the instep stitches so that you pattern stays centered down the top of the foot.
So depending on how many needles you're using, you may be taking two from each side of the sole circular and putting them on the instep circular, or taking two from each side of the single DPN holding the sole stitches (4 DPN), or taking two from one end of the first sole DPN and two from the other end of the second sole DPN (5 DPN).