Some of the tightening torsion is "locked in" due to friction under the bolt head or under the nut; if there is a lock washer under the head or nut it is even more locked. After a few load cycles the torsion usually relaxes and goes completely to zero. On some critical fastener applications the torsional stress is deliberately relieved by immediately turning the nut or bolt back slightly (around 1/8 of a turn) after tightening to specified torque. The studs may go to zero torsional stress more readily due to easier movement of the nut as opposed to the fixed head of a bolt or screw. This may be what ARP means.
Now why are "stretch" bolts used in connecting rods and flywheel screws? I have a theory having to do with the stress-strain diagram and achieving consistent and uniform clamping loads. I'm not sure though. Does anyone know why these bolts are torqued to yield? Cheers,
Jim