One of the things scientists look at when they look at the brain is its resting state, i.e., what areas of the brain are typically active when we're not doing anything in particular.
Why bother with the resting state? Because just like everything else that can be measured, our brains at rest display "normal" characteristics (i.e., population majority). Ordinarily, if scientist want to find out how brains differ, they have different types of people do things and analyze the differences. But if we can see differences between a normal Brain-at-rest with a population minority Brain-at-rest, we may be able to determine that they are part of that population minority without having to test their behaviour (just the behaviour of their brains when at rest).
For instance, in this 2010 Finnish study by Paakki et al., they found resting state anomalies in specific regions between ordinary teenagers (an oxymoron if ever there was one) and those diagnosed with Autism Spectrum Disorder (ASD). They found that the differences between ASD teens and controls was mainly in particular areas of the right hemisphere during the resting state. Some areas, such as the right insula, which has been associated with the ability to sense internal states and with empathy, had less activity, while others, such as the thalamus, which gates sensory information had more.
The promise of this technique is to identify psychological disorders at early stages to give treatment a better chance.
Now if only the treatments would catch up with the diagnostic tools, we'd be golden.