The flag ENCUTFOCK is no longer supported in VASP.5.2.4 and newer versions. Please use PRECFOCK instead (see Sec. 6.71.5).
The ENCUTFOCK tag sets the energy cutoff that determines the FFT grids used by the Hartree-Fock routines. The only sensible value for ENCUTFOCK is ENCUTFOCK=0. This implies that the smallest possible FFT grid, which just encloses the cutoff sphere corresponding to the plane wave cutoff, is used. This accelerates the calculations by roughly a factor two to three, but causes slight changes in the total energies and some noise in the calculated forces. The FFT grid used internally in the exact exchange (Hartree-Fock) routines is written to the OUTCAR file. Simply search for lines starting with
FFT grid for exact exchange (Hartree Fock)
In many cases, a sensible approach is to determine the electronic and ionic groundstate using ENCUTFOCK=0, and to make one final total energy calculation without the flag ENCUTFOCK.