A continuous tremolo on a string, while playing single notes on other strings. This technique is usually played with the Riz on an open string which sounds like a background drone and a melody taking shape with the single notes on the other strings, lower than the open one.
Playing Riz on an open string (G for example), start counting beats slowly. Then, in quite long time intervals (like every four beats) leave the Riz for a moment and strike the upper string (D, here) and immediately jump back to the Riz on the lower string again. The Riz itself should sound seamless with no gap caused by the single notes in between. Eventually, when you feel quite comfortable with adjacent strings, try to move the single picks to the farther strings like A or low G.
How to play Bam-riz