I have noticed that I have to make sure my fabric doesn't shift when I put the mat on...also make sure your fabric clears the blades...I've goofed quite a number of times.
tips with the twister: don't get frustrated and make too long of cuts, rock your rotary cutter to begin cutting, have your mat on a surface that you can easily either rotate or you can physically go around. I have saved my twister castoffs for future use. There is supposed to be a new tool coming out to use those scraps but the squares are easy to cut without a tool.