I admit I don't understand EHP.

What I think to understand regarding the diminishing returns is the following:

If you get hit by 100k damage and you use your 30% cooldown the damage is reduced by 30k so you get hit by 70k.
If you use the 20% cooldown the damage is reduced by 20k so you get hit by 80k.

If you use both together the damage is reduced first by 30% down to 70k and when you now use the 20% cooldown it's further reduced to 56k. So the 20% cooldown reduces 20k damage if you use it alone, but only 14k damage if you use it together with the 30% cooldown.

Please ELI5 it for me xD