The initiation complex makes a small gap for a helicase enzyme to bind. It is the helicase the 'undwinds' the DNA for most of replication.
In E. coli, for example, DnaA protein binds DNA to make a small gap between the two DNA strands, where DnaB enzyme (a helicase) can bind to the lagging strand (the one that is copied in fragments). From there, DnaB unwinds the DNA ahead of the polymerase enzyme.