When you add this post-function to a transition and trigger the transition on an issue, the add-on increments the selected field value by one. This is applicable to numerical fields only.
Jira sends notifications for the increment in the selected field value on the current issue. You can control the default value of the "Send notifications" option in the Configuration page under JMWE administration. Click here for more information.
A typical use case of this workflow is to increment a number field by one during a transition. Consider a use case where you want to track the number of times a bug fix was rejected by the QA team. To configure this:
Create a Rejection counter numerical custom field.
Add the Increase value of field post-function to the Reject transition of the Bug workflow.