STOP apparently was the creation of an MBSR participant a few years ago that has become a popular practice in MBSR training.
Stop. Stopping allows us to intentionally step out of our customary automatic reactivity first by becoming fully aware that being swept along in automatic reactivity is taking place at this moment.
Take a breath (or 3 breaths) (or 360 breaths!). Bringing conscious awareness to the breath allows us to continue the process of stopping, becoming stable in and present for this moment or these moments.
Observe the situation. Having stopped and brought full awareness to the present moment experience allows us to observe what is going on at this moment with greater emotional balance and clarity. We can be more objective and less judgmental.
Proceed. Having observed a situation with greater emotional balance and clarity, we are now in a better position to proceed to take action, likely in a wiser and more productive way than if we had not stopped but acted reactively and automatically. Note that sometimes we will determine, having taken the opportunity to stop and observe, that the best way to proceed is at this moment to take no action at all!
STOP is not to be confused with indecisiveness. It is not a cumbersome layer of mental subcommittees requiring interminable delays before taking action. The entire process could take place within a few seconds, depending on the situation. At other times it might require withdrawing oneself from an emotionally charged situation and sitting with the breath for at least a few minutes before proceeding. What it really does is greatly expand our options for how best to respond to any given situation.