Testing.

“Cut-off” is the financial statement assertion for classes of transactions, that they have been recorded in the correct period – so it’s all about the transactions around the reporting (year-end) date.
If inventory is received on or shortly before the y/e date it will be included in inventory (it exists and will be included in the count) – if the purchase invoice is NOT recorded until after the y/e, purchases and payables will both be understated, Similarly if goods are despatched before the y/e, revenue and receivables would be understated if the sales invoice is not recorded. So cut-off tests are taking samples of GRNs/GDNs immediately before and after the y/e and matching to purchase/sales invoices and confirming that they are recorded the correct side of the y/e.
An incorrect cut-off on purchases may also be detected by suppliers’ statement reconciliations. If the supplier sent the goods and invoiced it before the y/e but the audit client has not recorded it, it will appear as a reconciling item. The client may account for such items in a “goods received not invoiced accrual”.
I hope that provides some clarity.