“Illegal” in the sens that pieces are connected in ways not intended by the designers. An eqample here is the blac brick in th, middle of the engine with studs on either side. The black 1x2 plates with cheese slopes are not completely connected to it, so I would classify it as an illegal technique, since it would not be used in a lego set. Too fragile.

Here is a little infographic with examples I found: