Take a look at what that check box selected option actually says. "Allow creation of records via this relationship". In other words, selecting this option makes it possible to create records from the context of the related record. It does not make it an automatic process.
The typical use of this option is for when you have a one to many relationship and you enable the "allow..." option for the table occurrence on the "many" side of the relationship. If you then put a portal to the "many" table on a layout based on the "one" table occurrence, an extra, blank, "add row" will appear in the portal below the last portal record shown in the portal. If you enter data into a field in this "add row" FileMaker will create a new record in the portal's table and copy over the value of the match field of the layout's current record, thus linking the new portal record to the current layout record.
In most cases, there is no need to automatically create a related record in one table simply because you created a new record in the other table. That's because you can quite easily create as many related records as you need on an "as needed" basis.