Ok this is the Easton arrow tuning guild.
http://www.eastonarchery.com/downloads/tuning-guide
Walk back tune is just using your 20 yard pin at 20 yards 30, 40,etc. Using only the 20 yard pin with level in bubble perfect when shot and lined up perfect to see if the arrows group in one direction or not. Explained in manual.
Simply put some arrows might fly better than others especially if you have more than a few types.
Lawdwaz put it perfect in so far as form! I shot today horribly then before I left I concentrated on my form and hit 2 bulls at 20 then a decent group at 70. Practice shooting and your form before you start making any changes. And use proper arrow shaft and use all the same type all the time.
I shoot a 65 lbs bow and use a 340 ICS Hunter Pro at 8.8 gpi 29" draw they are fast and hard to pull out of 3D. This is the Arrow chart for spline selection. 400 shaft I have shot do not fly the same as the 340's, I see them bend when shot. Another thing is make sure your fletching is perfect! Any rip tear or odd shape will cause uneven arrow flight.
http://beman.com/products/ics-hunter-pro/