Okay, before you get nervous, we don't mean your extra large version...think about the smaller one-inch blade version instead. The technique is certainly not for the faint of heart, but many quilters swear it greatly reduces their "unsewing time" after making a piecing mistake. (Obviously, this is not for removing a quilting mistake.)
Skilled rippers have learned to hold the two fabric pieces in their hands so that the rotary cutter is held like a pencil or pen. One piece of fabric is held taut between the thumb and forefinger of your non-dominant hand, and the other fabric piece is held by the three remaining fingers of your dominant hand, pressing the fabric against your palm.
With both hands separating the seam that must be ripped, use the cutting blade (you can expose just the very end if you like) to "slice" the threads holding the pieces together. Continue to keep the fabric taut between your hands to expose the piecing thread.
If this isn't for you, then make sure the seam ripper you ARE using is sharp. Quilters tend to hold on to utensils way past their useful lifespan. Treat yourself to a new seam ripper if you have to work hard slice your threads...after all, you're worth it!