"Jumping the string" is a reaction to the sound your bow string makes upon release. When deer are startled they often spring down or duck then bound, if this happens it can cause your arrow to completely miss the target or hit higher then expected. My advise is don't worry much about the "jumping the string" thing, instead concentrate on the mechanics from draw to release, pick the hair(spot to aim) and follow through just like you do when punching foam. I have had very few deer duck or jump the string and those that did were usually of my own doing!

Practice shooting like you hunt and when its time to pull on a live target don't treat it any different that practice, you'll be fine!