While practicing my takeaway and back swing one afternoon about six months ago I noticed that by squeezing the bicep of my arm closest to the target (my target arm) against my pectoral chest muscle, minimized my margin of error on back swing and promoted solid ball contact and increased my accuracy with my swing. But I wasn't sure why. Yesterday while surfing the web I found a video on youtube entitled "Plane Drill" by Jeff Ritter that clearly articulated the same concept. He noted that by squeezing the logo on the front of your shirt with you arm on your and back swing and through swing created very similar results. Take a look.