Smartbolts have little discs in their heads that change color as the right amount of tension is applied to them. I recently installed a child safety gate with a similar mechanism: a red, spongy washer between the bolt and the frame; once the right amount of tension was on the bolt, the washer was squished down so much it disappeared and you could stop tightening.
Unfortunately, a torque-wrench does not measure bolt tension accurately, usually only about +/- 30%, because it does not take friction into account. The friction depends on bolt, nut, washer-material, plating, surface smoothness, machining accuracy, degree of lubrication and the number of times a bolt has been tightened. Fastener manufacturers often provide information for determining torque requirements for tightening various bolts, accounting for friction and other effects. However, in field applications, this information is often not available, practical or administered poorly.
(via Red Ferret)