"Swing" is the movement of the needle upwards,when you modulate the carrier.
If a radio,dead key says 4 watts,when you apply modulation it goes up maybe to a 7 watts,that the swing value. In the old days it was called,upward modulation
100% modulation occurs when you have 50% of the carrier power applied to the sidebands... 4w carrier plus 1 watt per sideband... Is there downward modulation where power is subtracted ??