What port scanning method involves sending spoofed packets to a target system and then looking...

From NMAP:

-sI Idlescan: This advanced scan method allows for a truly blind TCP port scan of the target (meaning no packets are sent to the tar- get from your real IP address). Instead, a unique side-channel attack exploits predictable "IP fragmentation ID" sequence generation on the zombie host to glean information about the open ports on the target.