Analysis on dungeon difficulty increase

[Jinglypockets]

In most boss fights of older expansions, most mechanics either have very obvious visual effects or a cast bar with the name of the boss skill, so one can use these as a clue to react. In this expansion, they added some skills that have no cast bar, name or obvious visual effects. This has turned verbal and quantifiable information into nonverbal and unquantifiable information. That is one technique to make it harder.

Can you give an example of a mechanic like this?