Send a Mail When a Field in a SharePoint List has a value of Yes

Based on your original details, this sounds like a pretty straightforward Flow.

Here is how I understand your question and the result you are looking for:

1. An item in a SharePoint list is modified

2. If a specific field value is changed to "Yes", then send an email.

You can accomplish this with the following steps in Flow:

1. When an item is created or modified (set this up for your specific list)

2. Condition (check to see if your field is equal to "Yes")

3. If the condition is true/yes, then use the Send an Email action

4. If the condition is false/no (your field does not equal "Yes"), then do nothing.

This Flow will run every single time that an item is modified in your list. However, it will only send the email if your field value is equal to "Yes". 

If there are more nuances or requirements to the Flow scenario you are trying to create, please include them here. Otherwise, based on the initial ask, I believe this will suit your needs.