A PWM (pulse width modulation) controller can be thought of as an (electronic) switch between the solar panels and the battery:

• The switch is ON when the charger mode is in bulk charge mode
• The switch is “flicked” ON and OFF as needed (pulse width modulated) to hold the battery voltage at the absorption voltage
• The switch is OFF at the end of absorption while the battery voltage drops to the float voltage
• The switch is once again “flicked” ON and OFF as needed (pulse width modulated) to hold the battery voltage at the float voltage