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: https://www.speedrun.com/crazy_chicken_the_good_the_egg_and_the_ugly/full_game?h=Any&x=824q363k and Moorhuhn Schatzjäger 3