When a card is placed in promiscuous mode, the card accepts every Ethernet packet sent on the network. This is quite taxing on the system but is a common practice for debugging purposes.
Also, when a card is placed in promiscuous mode, a network MAC address can be spoofed, i.e., the card accepts all packets whether they're addressed to it or not. Then on a higher (software) level, you can accept packets addressed to whomever you please. Promiscuous mode is disabled by default.