Ah, it's a text from some web review... not from avast! help file or web. (So, we can't guarantee it's correct.)
I'd put it this way:
avast! [resident protection] does not scan a directory when it's opened. That would slow down the system significantly, and there is not reason to do so (so, it cannot be enabled anyhow in avast!).
avast! can be configured to scan a file when it is:
1. started (executed) - that's the most important thing to do, and it actually should be quite sufficient
2. written to - that's the default setting for specific file types (executables). This makes it possible to detect an infected file as soon as it's created on the disk
3. read from - it is turned off by default. You can enabled this option, but it will slow down your system slightly. It may actually cause the following effect: you open a directory, using Windows Explorer - and Explorer reads specific parts of the files in folder to extract the icon and display it next to the file. In such a case, avast! detects this file access (reading) and scans the files - so you actually get the behavior you were asking about (the files in a directory are scanned as soon as the directory is opened) - but it's not because avast! would do it by itself, but rather because the file manager (Explorer) is accessing the files. As I said, however, this option is turned off by default and you have to enable it in Standard Shield configuration if you really want it.