Yes indeed, in most pulsed-field machines, the DNA molecules run zigzag- like along the gel. As this happens to all molecules in a given band, you nevertheless end up with normal-looking bands.

The reason for pulsing the field resides in the necessity for the molecules to re-orientate between the zig ad the zag. The longer a molecule is, the longer it takes, before it moves again along the electric field. Thus, large molecules will migrate slower through the gels than shorter ones.

Without this re-orientation phase , large DNA molecules could not be separated according to size. They would migrate at the same speed without separating. Once they migrate, they just pass the agarose network (the "pores"). It is somewhat funny to see that it is more the pauses between zigs and zags, needed for re-orientation before migrating again, that separate the molecules according to their length.