need help w/ unfamiliar syntax

Hi all, I've wasted two days trying to find this in the documentation. Google is no help here either; they only index text. While looking at some code using objects I came across this:

    $this->{$spec}

The example works with or without the curly braces, but I know they aren't there for just for decoration ;) Will somebody be so kind as to point me where in the docs this syntax is explained? tia