The Bb Wobble
aka Texas Wobble
This is quite simple, basically you finger an 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. On the tenor it should sound a bit like this:
Here it is on the alto:NB: For some interesting saxophone multiphonics, try playing this fingering without the octave key.