Rules with Manual trigger can be triggered via the ▶ button available from the rules listing page. Refer 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.
Click 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 how the input selector pop-up looks like. If no inputs are needed, a message to that effect will be displayed in the pop-up.