Pulsed field gel electrophooresis (PFGE) is used for the separation of large DNA molecules by changing periodically the electric field from a gel matrix. In a standard gel, DNA molecules bigger than 15 Kb move together regarldless of their size. PFGE is a varition that introduces alternating voltage gradient to improve the resolution of lager molecules.
The voltage is periodically switched among three directions: (1) To the central axis of the gel, (2,3) at an agle of 60 degrees from both sides. The result is that DNA does not move in a straight line through the gel but in an "net forward" migration pattern.