Item multiplying - How can you do that?

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
1. Jump
2. 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:

SnopSiana

Moorhuhn Schatzjäger / Crazy Chicken: The Good, The Egg & The Ugly

ElDominico