The fourth man high is the best risk in hockey

Sending a defenceman into the attack costs the odd break the other way and pays for it several times over.

Send the defenceman. Almost always, send him.

The activation, the fourth man arriving into the offensive zone below the top of the circles while his partner holds the blue line, is the play that most reliably breaks a good defensive structure, and it is still coached with a caution that belongs to an earlier era.

Here is why it works. A five-man defensive box in its own zone is built to handle three forwards and two point men. The three forwards are covered by three defenders, one defenceman handles the net front, and the coverage assignments are stable. When a defenceman leaves the blue line and arrives on the wall or in the slot, there is no fifth defender for him. Somebody has to leave a man, and the moment of decision is when the coverage fails, not the moment the extra player receives the puck. Half the time the pass to him never happens; the goal comes from the forward his arrival left unmarked.

The cost is genuine. If the puck turns over while he is deep, the other team is going the other way against one defenceman and whichever forward is highest. That is a real scoring chance against, and it is the play every coach remembers, because it looks like somebody's fault in a way that a failure to activate never does.

Which is exactly the bias I am complaining about. The cost of the activation is visible, specific, and blameable. The benefit is diffuse: a slightly disorganised coverage, a goal scored by somebody else three passes later, a shift that ends with a tired defensive unit unable to change. Coaches respond to blame, so the risk gets underused relative to its actual return.

The good version has rules attached, and they are not complicated. Activate when your team has controlled possession below the goal line, not off a loose puck. Activate on the side away from the puck so you arrive unseen. And the forward highest on the ice must swap upward as you go, so the blue line is never actually empty, which is a habit that separates the teams who do this well from the teams who do it occasionally and get burned.

Every structure has a number it cannot handle. Give it that number.