When playing Poppy, if you successfully negate the dash on Hecarim's E, he will be knocked back as intended, however, the damage will still go off.
1. Use your W as Poppy
2. Have Hecarim dash into you while your W is active (with minimal distance travelled)
I get that the damage isn't tied to his dash and that it is actually tied to his auto attack that gains bonus range based on distance travelled. I also get that at max distance travelled with his e, he can get up to 250 attack range which brings his attack range up to 425 which is longer than Poppy's 400 range on her W. However at short range, he can only really get around 50 - 100 range on his auto attack which isn't enough to bring his auto attack range above 400 however he still deals out his damage to Poppy regardless, meaning Poppy will essentially give him an attack range buff for blocking his dash... which makes absolutely no sense.
Hecarim sure is an odd champion.
EDIT: Apparently no one can read properly lol, I'm happy to be proven wrong but if you're going to bother commenting at all, read the ENTIRE THING PLEASE.
DISCLAIMER: I KNOW POPPY DOESN'T BLOCK AUTO ATTACK DAMAGE. THAT'S NOT THE POINT :)