Multiplying with negative is just turning around the direction of whatever you're doing.

Using the classic fruit example, adding gives you more apples, adding "negative apples" takes away apples. You can draw a card that says "you need to hand over 1 apple".

Now, adding 1 of these cards really just removes 1 apple. It's like a negative apple.

What happens if we remove 1 of these cards? You have 1 more apple than you had before.