If you are interested to get instant updates about a particular topic you can do so with Watch this topic option. However, this option is enabled only when you do not participate in the forum discussions.
In Projects, select the required project and click Forums. Click the Watch this Topic link at the top of the Right Panel. Notifications will be sent to you on latest updates for the selected forum topic.
If you wish to unwatch a topic, just click the Unwatch this topic link to stop receiving notifications for the selected forum post.