For those who don't know about this function... If you want to be sure someone knows you referred to them (or a post of theirs) you can use this and they are automatically notified.
Here's the info from the HU help section:
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How do I mention/tag other users in posts and replies?
When writing a post on HealthUnlocked, some users may want to notify another user of the post, either because they think it will be of interest to them, or because they think that user will have something to say about it. To do this, you can use our “@” mentions feature.
To notify another user that they are being mentioned, follow these steps:
1. Write a post. When you wish to mention another user, type the “@” character immediately followed by the user’s name, with no spaces: @username
2. As you are typing, a drop-down menu will appear with a list of users to choose from. Select the user you are trying to mention
3. Once your post is published, the user you have mentioned will receive a notification letting them know
Please note that you can only:
Mention users who follow the community you are posting in
Mention users in the body of a post or reply, NOT in the title of a post