The most reliable pressing trigger is not a pass or a touch, it is the moment a receiver turns his body the wrong way.
Press the hips, not the ball
Ask most players what starts a press and you get an event. The pass back. The ball travelling to a full back. The heavy touch. All of these are real triggers and all of them share one weakness, which is that they are visible to the man on the ball as well, so he has already begun solving the problem before your first stride.
The better trigger is the body. Watch the receiver in the half second before the ball arrives. If his hips are closed to the touchline, he cannot turn forwards without a second touch, and that second touch is the time you are being given. Nothing about the pass told you this. The pass may have been perfectly played. The receiver simply set himself badly, or a teammate positioned behind him made turning pointless, and for a moment he is playing with half a pitch.
The reason this is better is that it cannot be disguised. A team can be coached to avoid the obvious triggers. Do not pass back under pressure, do not play into a full back on the touchline, keep the goalkeeper involved so the shape stays stretched. Body orientation is not something you can hide, because it is determined by where the player has to look to receive.
It is harder to teach, obviously. An event trigger is a rule and rules can be shouted from a sideline. A body trigger is a read, and reads require a player to be watching a man who does not yet have the ball, which is precisely the habit most defenders were trained out of as children.
The teams that press best have four or five players making that read simultaneously, and when it works the press appears to start before anything happened. That is what people are describing when they call a side telepathic. There is no telepathy. Everybody looked at the same pair of hips.
I would add one caution. Reading hips against a genuinely two footed midfielder who can receive on the half turn either way is a good way to sprint into empty grass and leave your midfield short.
Which is roughly what a player like that is worth, and why he costs what he costs.

