imagine each dozen divided into three horizontal lines.
trigger: wait until one dozen hit one of the three horizontal lines two times, (or three times for a stronger trigger) then bet the two unhit lines. (8 numbers bet).
negative progression 8 number = 1. 1. 1. 1. 2. 2. 3. 4. 5. 6. 8. 10. 13. 17. 22