In The Financial Edge our Journal Entries must balance by period.  Therefore when entering Debits in one period and Credits in the next we must have an opposing entry in each period to balance. 

For Example:
We have a manual Journal Entry we have created that reflects DR to an expense account in December.  In the same Journal Entry we have a CR to the cash account offsetting these expenses in January.  These transactions cross funds. 

It is necessary to enter an offsetting entry in each period to balance the period.  Therefore we must use CR another account in December for the amount of the Expenses and we must also DR this same account to offset the CR in January.  This will result in this account washing itself leaving us with the same results. 

If we do not enter these balancing entries there will be a remaining balance in the interfund or Due To Due From accounts that does not resolve until January.