The intent of this protection is to avoid unintentional writes to an attacker-controlled FIFO, where a program expected to create a regular file.
When set to “0”, writing to FIFOs is unrestricted.
When set to “1” don’t allow O_CREAT open on FIFOs that we don’t own in world writable sticky directories, unless they are owned by the owner of the directory.
When set to “2” it also applies to group writable sticky directories.
This protection is based on the restrictions in Openwall.