This technique is an easy way to make a curved pieced seam in your block. You can easily free hand a curve that you want into any block. You probably won't cut and sew a precision piece with this method, but it would be perfect for an improv block!
Step one: Lay your block on top of the fabric which will become your inside curve. Both fabrics should be right side up. Make sure there is at least a quarter inch over hang on all sides.
Step two: Using your rotary cuter, slice a few inches into both fabrics where the curve will be. Holding on either side of the fabric where you have just cut, continue the curve. This ensures the fabrics are cut at the same time and will be less likely to shift. If you need to, and depending on the size of the curve, keep moving along, holding the fabrics in place until your curve is complete.
Step three: remove the excess main fabric (the bit you just cut off), pick up the inside curve fabric and flip it over onto the main fabric. This will be your seam. Gently sew along this seam, trying not to stretch the fabric. You will find they should ease into each other.