It says in Law 12, under "Cautions for unsporting behaviour", that:
A player must be cautioned for unsporting behaviour including if a player: [...] uses a deliberate trick to pass the ball (including from a free kick) to the goalkeeper with the head, chest, knee etc. to circumvent the Law, whether or not the goalkeeper touches the ball with the hands.
I was wondering if a deliberate high pass from one defender to another, who then heads it back to his keeper, is considered to be a trick.
What you have described it not a trick, nor is it illegal.
The first player has simply played an aerial pass to another defender. This is not a deliberate trick to circumvent the Law.
The second player has simply headed an aerial ball back to the goalkeeper, at a height that a player would be reasonably expected to head the ball. This is not a deliberate trick to circumvent the Law.
A deliberate trick to circumvent the Law would, for example, include, but not be limited to:
Any team that attempts the scenario outlined in your question this puts themselves at a considerable disadvantage. A poorly-skilled team would possibly fail to make a high pass at the right height, or the header would be difficult for the keeper to get to. This could see a loss of possession in their defending third.
At the highest levels of the game, even if the high pass was made perfectly and the header was directly to the keeper's hands, a defending striker would have a good chance of intercepting the high pass (since they generally move slower) or the header back to the goalkeeper. Either way, the defender making the header would be under significant pressure.
Deliberate trickery to circumvent the Law is rare enough that it is obvious when it occurs. Referees should not go hunting for it.
