VMdirectPath is a form of PCI Passthrough to give direct hardware access to a VM. It is usually used (on supported hardware) to give access to I/O card (network and storage) for maximun (native) performance.
This is supposed to increase performance in the virtual machine using VMDirectPath, as now the CPU is not doing network device emulation.
http://blogs.intel.com/idf/2008/08/io_pass-through_lets_us_have_o.php
http://kb.vmware.com/kb/1010789
