What is a market structure shift (MSS)?
In an uptrend, the last higher low is the level buyers must defend. When a fast, large move closes below it, structure has shifted. ICT adds a condition SMC courses often leave out: the break must come with displacement, so a fair value gap is left behind.
The usual sequence is a run on liquidity first — a new high that takes buy stops — then the shift down, then a retracement into the gap where the short is taken. The bullish version is the same upside down.