'Combustion' - draws fire in selected area (as in 'Flame' gradient 'FC->BC->transparent' colors), which burns only in pixels with selected color. For example, you can draw circle with fire, which used in circus when lion jumps over it: Draw gray cricle. Set red FC, yellow BC. Execute 'Combustion', set gray circle color as color for combustion. Next set parameter 'Base noise' which sets dispersion of fire length in different parts of flame (set 20). Then you can select where flame will burn - only around of our gray circle, or around and inside of circle gray pixels. Then set Effect force (15 for example). That is all =)