Side-Flutter Tips
The side-flutter is where your tongue moves side to side... apposed to on and off for the flutter. There's three common ways to approach this.
1) Your tongue stays over three holes in your standard four-hole tongue block embouchure. For example, your tongue is at left and the 4 draw sounds... then it moves to the right and the 1 hole sounds... and then to the left and the 4 draw sounds... rinse and repeat. In this version you hear single notes bouncing back and forth.
2) Same as above, but you use more of the tip of the tongue and you get more of a chordal sound due to the fact that your tongue only blocks about two holes.
3) You don't slide the tip of your tongue left to right, the middle of your tongue moves side to side... not the tip, but the middle, like an exotic hip-shaking dance of the tongue. I can't do this, but when Joe Filisko showed me it was a sight to behold!
By the way, Sonny Terry also used this, but his lips where over three holes, blocking two holes (bouncing back and forth between 2 and 4 draws for example).