There are two types of mechanisms, some are dual flush and give you the option of choosing between a full or half flush accordingly to your needs, while others are single flush which means once they’re activated they can be interrupted by pressing the flush button again.
Whenever possible choose a dual flush mechanism to save water.
When we press the flush button the outlet valve opens and causes the water to flow out of the cistern down to the toilet. The float in the inlet valve goes down as the water comes out of the cistern and, as it goes down, it opens the inlet valve and the cistern begins to fill. The float follows the rise of the water and when the cistern is full the inlet valve closes.