*Edit to add: Those guys are doing free fall jumps. Didn't do any of those in the military.
For static line jumps the rucksack is attached to quick releases on the harness under the reserve. It's over your thighs basically. Once you descend to the correct height you release the quick release straps which causes the ruck to drop 18 feet or so below the jumper but is still attached by a tether. At night you can often hear your ruck hit about 1 second before you do.
*Edit to add: Those guys are doing free fall jumps. Didn't do any of those in the military.
For static line jumps the rucksack is attached to quick releases on the harness under the reserve. It's over your thighs basically. Once you descend to the correct height you release the quick release straps which causes the ruck to drop 18 feet or so below the jumper but is still attached by a tether. At night you can often hear your ruck hit about 1 second before you do.
For free fall, I don't know if ruck position in front or behind is jumper's preference or weight driven.