Slow breathing with a longer exhale shifts the balance of the autonomic nervous system toward the calming side. It lowers heart rate and reduces the jitter that makes fingers fast and judgement poor. The four-count square is one common pattern; the specific counts matter less than the pace and the extended exhale.
It is most useful at a defined moment rather than as a general practice: after a loss, before a scheduled event, or when you notice a physical spike. Two minutes is enough to change the state you make the next decision in.
It is a state tool, not a strategy. Calm makes a bad trade no better, which is why it belongs inside a tilt-protocol that also stops you trading for a while.
Related: emotional-regulation, tilt-protocol, arousal-regulation, stress-response