pick one random doublestreet (or track the last hit)...then wait for this ds to be hit twice (two different numbers). ideally it should pass atleast 10 spins before this event. after 2 number have been hit , bet the 4 unhit numbers.
could be played flatbet, perhaps? (haven't tried that)
i use progression +1u after 9 spins, and after hit, i use the same bet for the next trigger.
for an example once win with 3u , bet for the next trigger is 3u (reset progression when you reach a new high)