AKA Bb Wobble or Texas Shake
This is a stunning effect used by many of the greats including Ben Webster, Illinois Jacquet, Earl Bostic, Arnett Cobb to name but a few.
It is really very simple if you know the fingering: basically you finger an upper register Eb but without the LH 3 G key:
You then need to blow quite hard with an exaggerated vibrato so that the note alternates between G and Bb with a yodelling effect.
It should sound a bit like this:
If you are having difficulty, then try the vibrato/bending exercises first and aim to be able to lower your note a lot.
For some interesting saxophone multiphonics, try playing the fingering for the wobble but without the octave key.