Substantive procedure: Cut -off assertion of revenue

The followings are the substantive procedures for cut-off assertion of revenue on page 314 BPP AA Workbook:
1/ Perform analytical procedures on sales returns, comparing the ratio of sale returns to sales
2/ For sales return, select a sample of returns documentation around the year end and trace to the related credit entries.
I do not find any relationship between the sale return and the cut-off substative procedure of revenue as cut-off assertion refers to time of recording the revenue.
Please explain more to me about that.