That small press of your own finger does two jobs. It marks the thought, and it gives you a bit of contact with yourself while everything around you is loud.

What the research says

Across imaging studies, heart rate variability lined up with activity in the brain circuits that manage a threat response, which supports using it as a readout of how well someone is regulating. That gives good reason to treat bodily settling as more than a mood. It says nothing about self touch in particular, which has not been tested this way, so the finger press earns its place as a marker first and a comfort second.

Strength of evidence: one line of research. A single study, or a theoretical account.