DFD stands for Direct From Disk and is a technique for playing back large and very large instruments and samples without loading them entirely into RAM. In fact, only the first portion of each sample is loaded into RAM permanently; the rest is read from the computer's hard disk while playing the instrument. RAM is able to react virtually instantly, delivering the first portion of any sample the user requests, while the computer goes to fetch the next portion of that sample from the hard disk. With DFD switched on you can load samples with up to 2 Gigabytes each even with moderately equipped computers. Native Instruments software does support this technology.
For detailed instructions, please read the attached guide "DFD Demystified".