What is the principle of magnetic bead-based normalization of DNA libraries?

The principle of magnetic bead-based normalization of DNA libraries is that a given volume of beads can bind a consistent amount of nucleic acid molecules. If each library contains enough molecules to saturate the beads, an approximately equimolar quantity of library fragments will bind and be retained from each sample. On washing away unbound molecules, only the bead-bound molecules will be left behind to represent each library.  

One advantage of magnetic bead-based normalization is that it eliminates the need to quantify and dilute samples prior to pooling. Another advantage is that it produces more consistent read depth compared to most other existing quantitation-based methods.  

The downside is that the number of molecules in each library must necessarily equal or exceed the binding capacity of the beads. The excess is discarded, which can be wasteful. If your sample is precious or in short supply, using qPCR-based quantitation may be a better option.