With io-net network driver arguments could be used to enable promiscuous. Since io-pkt the promiscuous is enabled on demand by the stack. Setting up a bridge or running tcpdump (bpf utility) enables this mode for example. Note that nicinfo maybe prints inaccurate status of the promiscuous mode. Check the status with ifconfig instead.
This is an c-example on how to enable promiscuous mode on interface en0:
PROMISC in the ifconfig output for the en0 interface indicate that promiscuous is active.
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 33192
inet 127.0.0.1 netmask 0xff000000
> mtu 1500
media: Ethernet 100baseTX full-duplex
inet 10.42.171.102 netmask 0xffffff00 broadcast 10.42.171.255
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