Can openers for Italy and Germany, I can speak to. The concept is as you stated: a teammate, who goes before you, clears out blockers so that the main force can rapidly advance. Italy can “can open” for Germany because (usually) there are no ANZAC or French forces in the Eastern front. Russia can’t put single inf in territories to try and stop the German mobile units from two step advances because the italian force will kill it. The Russians must either put many units as blockers, which Germany will gladly kill since fewer units now protect Moscow, or retreat and turtle, which is also good.
As you get closer to Moscow, having the Italians take a territory may not allow 2 step advance, but the Germans can now reinforce with aircraft, which often prevents a Russian counter, and so the German advance continues.
Incorporating the Italians into your Eastern front is a key enabling tactic for the Axis.
As far as a British “reverse can opener”… as i see it, you are right, this shouldn’t work for helping the Russians, since Germany moves first and would avoid any counter attack. The US can can open for Brits.
Perhaps, a reverse can opener is reblocking what the can opener forces cleared. In which case, you would need ANZAC or French forces to take out the Italians and establish control/ block. The Brits don’t move at the right time to make this happen.
