Measuring the torque in the untightening direction is referred to as the break loose torque and is less than the residual torque for a newly tightened fastener. When untightening the bolt/nut, a lower torque than the tightening torque is needed since the thread extension torque component assists in the untightening. You can think of this, in a way, as it is easier to push an object downhill than it is uphill.
The term residual torque is usually used but it is somewhat a misnomer since there is little or no torque present in a tightened fastener. The torque measured is a function of the preload in the fastener, the thread geometry, and the friction conditions present.
Normally the movement of the fastener is sensed by the person completing the test. Although not widespread, there are torque audit wrenches available that measure the torque value after a couple of degrees of rotation. Using such a wrench removes the human judgement involved. Alternatively, the fastener and joint can be marked, and the nut or bolt head rotated until movement is noted. One issue that can arise on longer bolts is that if the nut is only rotated by a small amount, the friction grip between the nut and the surface is broken without overcoming the friction grip between the threads resulting in a significant measurement error. There are several factors that influence the reliability of the method in determining that the measured torque is a true representation of the actual value.
One common reason for completing residual torque measurements is that some concern has been raised about the integrity of assembled joints. This may be that a loosening issue was experienced on one or more joints, or maybe, bolt fatigue failures have occurred. The longer the time between completing the residual torque measurement and the installation date, the greater will be the uncertainty that the torque measured will be a faithful representation of the bolt preload. Over time, on most joints, especially those without a grease having been applied, friction tends to increase. Such a friction increase results in a higher torque being needed to rotate the fastener. Hence, this can mask that the bolt preload has dropped and is below acceptable limits.
Having said the limitations of the approach, often residual torque measurements can provide crucial information on whether a larger structural integrity issue is present. Usually, there are few alternatives to completing such checks to decide on what, if any, action is needed.
The reasons why the bolt preload can reduce over time can be due to several effects. But perhaps this is left to another newsletter.
Bolt Science have a couple of online training courses on the technical aspects of bolting, further information of this is available here.