The extract created from the leaves of the hazelnut-bush-like-magic-tree, commonly called Witch Hazel. We have gone into detail about Witch Hazel in cosmetic products here (it's astringent, soothing, antioxidant and antibacterial), but the important part to know about the leaves is that they contain much, much less active components than the bark. In fact, it contains hardly any tannins (only 0.04%) and the most active component in the leaves is the antibacterial gallic acid.
Too many tannins can be very astringent and irritating to the skin, so this is not necessarily a bad thing. Even the small amount of active components in the leaves seem to give it nice soothing, astringent, and antibacterial properties.
Show me some proof
- Wang, Huafu, Gordon J. Provan, and Keith Helliwell. "Determination of hamamelitannin, catechins and gallic acid in witch hazel bark, twig and leaf by HPLC." Journal of pharmaceutical and biomedical analysis 33.4 (2003): 539-544.