Multiphonics are really splitting the air in your saxophone. The result is several different notes speaking often there is a low and several different higher notes. I got to see Tommy Smith, if you don't know him you should check him out. I know he has a website but I don't know the address off the top of my head. But anyway, I got a chance to see him live and one thing he did, besides playing amazing all night long, was playing a line with multiphonics, it was really cool.
If you are really into multiphonics give this a try. . .
play a low C with out your F key
Low B with out your E key
This have a similar collection of notes a halfstep apart. Also try,
Low Bb with out your C key
Low B with the C key down, but Eb key open
There is a pattern to the multiphonics where there is a distance between the number of keys you have down and where you have a open or vented key. Good luck, it is an intersting thing and drives most other people nuts. If you play any classical saxophone there are several pieces that include multiphonics and give fingerings for them. . . . any way, I hope this helps with your question.
Keep practicing and listening