Teams do not press constantly, because doing so is physically impossible and tactically wasteful. They press on cues, and the most dependable cue is a pass played towards the passer's own goal.
A backward pass buys the pressing team time
While the ball travels backwards, the pressing team advances forwards, so the distance closes at the sum of both speeds.
By the time the receiver has controlled it, opponents who were ten metres away are on top of him, without anyone having sprinted from a standing start.
The pass has effectively invited the press by moving the ball into the space the pressers wanted to reach.
The receiver's options are already limited
A player receiving a backward pass is usually facing his own goal and must turn before he can play forward, which costs time he does not have.
He also has fewer teammates ahead of him in useful positions, since the team's shape was built around the previous ball location.
Pressing him is therefore lower risk than pressing a player who received facing forward, because being beaten is less likely.
Other triggers work on the same principle
A slow pass, a ball played into a player with his back turned, a heavy first touch and a pass to a weaker foot all create the same temporary inability to progress.
Pressing schemes list these explicitly so that the whole unit reacts to the same event rather than to individual judgement.
Shared triggers are what make a press coordinated, since eleven players pressing on eleven different cues produces gaps rather than pressure.
The press has to be led from the front
The first presser determines the direction of everything behind him by which passing lane he blocks with his approach.
If he closes the inside, the team shifts to trap wide; if he closes the outside, they compress centrally. The unit reads his angle rather than waiting for a call.
A front player who presses without setting an angle causes the players behind him to guess, which is a common reason presses appear disorganised.
Recognition of the non-trigger matters equally
Just as important is knowing when not to press, since pressing a player who can turn easily leaves the pressing team stretched with nothing gained.
Teams that press well are noticeable for how much of the time they are not pressing, holding shape and waiting for the cue.
That restraint is harder to coach than the pressing itself, because it requires a player to stand still while an opponent has the ball in front of him.