row hash locks

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row hash locks

When tpump update a row, I understand that it does a row hash lock. Based on this, I suspect that this means that all the rows with the pi values that hash to the same value as the pi being updated will be locked.
(Please correct me if I'm wrong) I can see where this may cause blocking.

When tpump does inserts however, I don't understand why there are blockers.

Can someone explain this?
Thank you.