For a bearish momentum: It is when price action makes a series of lower highs(LH) and lower lows(LL) and then starts to reverse. This is noticed with a breach of the last LH i.e. the last LH fails to make a new LL and instead a candle closes above it to form a higher high (HH) causing structure shift.
For the trend change to be validated, the last LL that caused market structure shift (MSS) must hold when retested (formation of HL and HTF entry zone in the diagram).
Note, after identifying trend change on high time frame (HTF), you must wait for MSS to happen on a lower time frame (LTF) e.g. MIN 30, 15, 5, etc, and find an entry with a simple invalidation.
The waiting period for price action to develop requires alot of patience and that is where lots of traders f*vk up!
Obvisouly, you must find other confluences that validates your entry zone such EMAs, HTF support, volume, liquidity, overall market trend etc.
The opposite is true for a bullish momentum but in this $UNI example a trend change after a prolonged bearish momentum is illustrated.