Rules with Manual trigger can be triggered via the ▶ button available from the rules listing page. Refer to the below screenshot.
If ▶ button is not available for a manually triggered rule, which means there are no actions defined within the rule or that the rule is disabled.
Clicking on ▶ opens the pop-up that expects input/s from the end user. Once the input/s is/are provided & user clicks on Ok all the actions within the manual rule will be executed. This is what the input selector pop-up looks like. If no inputs are needed, a message to that effect will be displayed in the pop-up.