In ConfigMgr 2012 every collection has a checkbox Use incremental updates for this collection. This option is to periodically scan for only new or changed resources from the previous collection evaluation and update the collection membership with only these resources. This is what in ConfigMgr 2007 R3 was called Dynamically add new resources. The biggest difference is that now the interval can be configured.
The Collection Membership Evaluation Component Properties popup will show. Here the interval can be configured. Notice that the default value is, just like the delta discoveries on the Active Directory, 5 minutes. In general there is no use in running the Collection Membership Evaluation more often then the delta discoveries on the Active Directory. As a collection membership is mostly based on a membership and/ or existence in the Active Directory.
Now after a change of the interval to, for example, 2 minutes, there is an easy way to check and see if the change got applied. This easy way is to look at the colleval.log. In this case, before the change the Incremental Cycle is triggered every 5 minutes (default).
And in this case, after the change the Incremental Cycle is triggered every 2 minutes.
Note: The best practice threshold for collections, with use incremental updates enabled, is about 200. For all the best practices for collections, see: http://technet.microsoft.com/en-us/library/gg699372.aspx