Zetsu is able to plant spores in his opponents' bodies, unbeknownst to the opponents themselves (including Shinobi of Kage-level). These spores are time-delayed and, once they're activated, grow into clones of White Zetsu. The clones proceed to restrict the targets' movements and absorb their chakra. The absorbed chakra can then be transferred to another individual, provided the clones of White Zetsu were able to make contact with the person whose chakra is to be replenished.
