First, its just a convenient name for using combined forces, turn order, and the rules to your advantage. It has multiple permutations.
The general rule that a single unit can block COMBAT movement through any land or sea territory
There is also the consideration that you can liberate a straight or territory with one power, then pass through it or land airplanes there with the power following in turn order before the enemy can react.
Lets say we have 2 Italian tanks and 2 Italian Mechs in Poland, with 15 Germany Tanks.
Russia has 1 infantry in each of Baltic States and Poland.
If its Germany’s turn, he can’t simply pass through these blockers, he has to eliminate them and stop. But lets say that UK has just taken its turn, and its Italy’s turn. Because Italy is near the end of the turn, and Germany at the beginning, the Germans will get to go before the Allies do, and the Axis players will act twice in succession (interrupted only by France).
So, you declare war with Italy, attacking both Eastern Poland and Baltic States with 1 armor and 1 mech each. If you kill these blockers, even if you don’t take over the territory, when the German turn comes up, they can blitz straight through these spaces and attack any Russian stacks hiding in Novogrod and/or Belarus. The Russian player may have assumed that these units are safe, but once Italy destroys them, Germany can move through.
This has many permutations, and it can be done with air, sea or land units in various ways. However it is done, it is an example where one power “opens the door” (for example, USA taking Denmark), then a power that follows in turn order “walks through the door” (then UK invades Germany).
Most people when they are referring to the “Italian” Can Opener mean that they have got a force of mobile units that roam around threatening these blockers, so that the defending player cannot stand in force on the front line, and he is also afraid to stand in force behind it, either. The Italians only start with 2 fighters, 1 bomber and 2 armor that they can use this way, but they can build more (tanks and mechs)
The tactic can be used in many situations, for example, an USA bomber could blow up the single destroyer blocking SZ 16 so that ANZAC can move through and attack SZ 6 on ANZAC turn, since ANZAC follows the USA in turn order but Japan does not go between.
This is entirely based on the turn order, because if the other side gets to go between your teams, they can simply replace the blocker or adapt to the changed situation. It is a set of moves and tactics that takes advantage of the separate turns of each player to kill blockers and make the back lines of your enemy vulnerable, which forces them to play more conservatively (making them stack in the backfield, rather than leaving SZ 6 empty knowing it cannot be entered by blocking SZ 16)