FvwmButtons ans swallow

Please explain exactly how swallow directive works ?
In another worlds how could I swallow YYY to FvwmButtons. ( How the commands and materials work in the Fvwm swallow .)?

This is an example of how I use it:

grep Swallow Buttons.fvwm *FB-Desk: (1x2, Swallow (UseOld,Respawn) 'FvwmScript-DateTime' 'Module FvwmScript $./scripts/FvwmScript-DateTime') *FB-Desk: (1x9, Swallow (UseOld,Respawn) 'FvwmPager' 'Module FvwmPager 0 8') *FB-Desk: (1x3, Left, Swallow (NoClose, UseOld) 'stalonetray' 'Exec exec stalonetray') *FB-Desk: (1x28, Swallow (UseOld,Respawn) 'fIconMan' 'Module FvwmIconMan fIconMan')