First, accept that a negative is the inverse, or opposite of something.

We can intuitively understand that multiplying slices by a negative inverts them - turns them from slices given into slices taken away. (Distribute the multiplication into addition if necessary to drive the point home.)

So it follows that multiplying negative slices by a negative inverts them again, turning them back into slices given.

It’s not a convenient visual, but if a negative slice represents the “taking away” of a slice, then a negative negative takes away the “taking away.”