There are many different positions a sniper and his rifle can get into to make a shot. The underlying thought process is that a sniper might have to stay in whatever position he sets up in for an extended amount of time. Whatever position it is needs to be stable, somewhat comfortable and without muscle tension to be able to hold the rifle on target. This is assuming that the rifleman has time to set up a position and is not taking a snap shot.
Pack supported prone is much like shooting off a bi-pod or other supporting structures. Your body is as flat as possible on the ground, lined up straight behind the rifle and the shooter should be able to relax his body so none of his muscles are tensioned. The pack itself makes for a great support for the rifle. It can be moved around, adjusted into almost any position, dented in the middle or bunched up to adjust for elevation or windage. Not enough time to pull down the bi-pods, just throw your pack down and use it for the rifle support.
Most packs have some type of camo printed on them and can aid in concealing the shooter as he lays behind it. The pack is easy to adjust, provides some distortion of the shooter profile and has all your gear right in front of you for easy access to individual equipment needs.
A video presentation of the pack supported prone is available on ATAC TV.
Lawrence and Tom explain the fine details of the position in the attached video linked below. Watch the video and try the position at your next firearms training session on the range. It works great and gives you an alternative position.
Watch on ATAC TV Here: Pack Supported Sniper Rifle Training