Pivoting uses a compromised host as a relay to reach otherwise inaccessible network segments. Metasploit route command, SOCKS proxies, SSH -L/-R/-D forwarding.
Meterpreter: run post/multi/manage/autoroute → route add [target subnet] 1 → use auxiliary/scanner/... to scan through the compromised host. Proxychains routes tools through SOCKS proxy. Essential technique for reaching internal-only systems from an initial external foothold.