Do you mean when the ruler shifts? If so, I would suggest you walk your hand along the ruler as you cut:
Hold the ruler close to you where you are going to start the cut. Cut along the ruler edge until the rotary cutter is about even with your fingertips. Hold the cutter still, then walk your other hand up the ruler, so it is just past where the cutter is. Repeat. I hope that makes sense. With practice, it goes very fast.
If the fabric is curving near the selvedge, that can be easily remedied by cutting the selvedge off before cutting your strips.
I hope I have helped.