Think of it as a very fast light bulb dimmer. The power will vary from no light to very bright. The act of turning the dimmer knob would be the 'modulation'. The brightness of the bulb would be radiated RF power.
To jam AM you have to 'swamp' the reception area with a HIGHER power steady state RF signal. One with no modulation or with a source of pink noise to modulate the jamming signal.