What does Guppy's paw do in wrath of the lamb?

I got Guppy's Paw and I do not know what it does..


You can activate it in order to convert one of your red heart containers into three soul hearts. To quote the BoI wiki:

Upon activation, one of Isaac's heart containers (if any) will be replaced by 3 Soul Hearts. (In Eternal Editon it only gives you 2 soul hearts)

Consuming your final Heart Container will not kill Isaac.

The paw removes full hearts first, leaving you with at least a half normal heart before the rest of your heart containers are used up.

If used with half a normal heart remaining, the player can convert their hearts quickly, effectively tripling their health. This is especially useful against the final bosses. (Mom, Mom's Heart/It Lives!, Satan, Isaac, and ???.)

[Source]

As one of the Guppy items, it also counts towards the character's transformation into Guppy, which is a powerful effect if you can collect three Guppy items in a single run.