An MSS is a break of structure made by an impulse rather than by drifting. The break mechanics are identical: the level has to be broken on a close. MSS adds a requirement about how price got there.
Two conditions, both required
The leg that broke the level counts as impulsive when both conditions hold at once.
A span of at least 1.5 ATR. The whole leg is measured — from its start to the break bar, not the height of the breaking candle itself. The leg's start is the extreme the move set off from: for an upward break, the lowest price between the reference pivot and the break bar.
A gap inside the leg. A same-direction FVG must fit entirely within the stretch from the leg's start to the break bar. A fast move almost always leaves an untraded stretch behind it; if there are no gaps, price moved tightly, and that is not an impulse whatever the span.
The second condition filters out legs that collected their 1.5 ATR over a dozen quiet bars.
What it looks like on the chart
An MSS is not drawn as a separate object. It is a label: a break that passes both conditions is annotated "MSS" instead of "BOS" or "CHoCH". It still remains either a with-trend or a counter-trend break — impulse does not cancel direction.
In other words, MSS is not a third kind of break but a property on top of the two that exist.
Why the property matters
It decides whether an order block appears. A block is looked for not before every break, only before an impulsive one: the leg needs a gap inside it and a span of at least 1.5 ATR. On top of that the break needs a significance shelf of at least two degrees — the event has to be visible on more than one scale.
Without that filter, blocks would be found before every other bar and would mean nothing.
What MSS does not mean
Impulse is a property of a stretch already travelled: price crossed it fast and left gaps. It is not a promise of continuation and not a signal. On the chart the property exists precisely to explain why a block sits here rather than somewhere else.