How to get 3D effect when flipping an element?

Question

css

Such cases: ptosnm.ru/ru/issues?year=2014
When turning the plate over, its colored part goes beyond the borders - one side increases, the other decreases.
I was only able to achieve a "flat" flip - both sides are always the same height and it feels like the element is shrinking rather than flipping.
I scoured the css on this page for a long time, but did not understand PREKOLA.
What creates such an effect?

Answer 1

The perspective property of .month-issues .month-issues-item
To disable 3D, you can write transform-style: flat;
Or set the perspective to a very high value.