It's very simple. Item multiplying works on all items that are hidden in all kind of boxes, like the wooden ones
To perform this bug and get for an example two hearts instead of one from a box
- Jump
- When you're about to hit the box from above use punch at the correct time
Try it yourself : D If it worked, congrats. You can now have double ammo, double hearts and double points from boxes
And yes, it does work on boxes that has slowdowns, speed and invincibility power ups but the power ups don't stack so you cannot have them for more time than it was intended
This also works in: Moorhuhn Schatzjäger 2: https://www.speedrun.com/moorhuhn_schatzjager_2?h=Any&x=n2ymy472 and Moorhuhn Schatzjäger 3:
The current timing rules are inaccurate, so I'm changing them to be more specific.
"pressing "Start" or "New Game" in the main menu" is not a visible action. Instead, the run will start at the first frame of the the graphic that's shown after starting a new game.