“We can really see the weaknesses of a body when a person is becoming tired. Sometimes we have to bring the body to its limits for physical or psychological weaknesses to appear.
This need not take a long time; we just have to put the person in an unusual position. Someone who can easily sit in Daṇḍāsana, for example, could be asked to lean backward.
Someone who can do Utkaṭāsana could be asked to squat with one foot slightly in front of the other, comparing the two sides.
Breathing can also be used. It is possible to save time by asking someone to add special breathing requirements to their Āsana. They will be concerned about these and problems in the body may appear faster. For example repeating Uttānāsana twelve times with a 15″ inhale.”
– TKV Desikachar