As you can see from the video, it's taken from Linda Perry's solo album, In Flight. This was one of Grace's last ever times in a recording studio and it was done in 1996. Linda was a friend of China's and invited Grace to sing on Knock Me Out. Grace says she was honored by the invitation as she rarely has ever been invited to guest on other people's albums or sing backup. Knock Me Out also appeared on the soundtrack for The Crow: City of Angels.
A few rare instances are her backup vocals on David Crosby's If Only I Could Remember My Name album, Ned Lagin's Seastones and Heart's 1985 self titled album where Grace along with Mickey Thomas sang backup on What About Love.
"I know it's hard to defend an unpopular policy once in a while." - Zappa