Run-off cover does not vary greatly from a standard PI policy. To understand how run-off cover functions, you first must understand how claims-made insurance policies operate.
Run-off cover, like PI insurance, operates on a claims-made basis. Claims-made policies cover claims made during the policy period, regardless of when the incident or negligence took place. Claims-occurring policies, on the other hand, cover incidents or negligence that occurred during the policy period, regardless of when the claim was made.
The difference, then, is that claims-made policies provide cover for when the claim is made (meaning a policy will cover claims so long as they are made during the policy period), while claims-occurring policies provide cover for when the incident occurred (meaning a policy will cover incidents that occurred during the policy period, even if the claim is made years later after the policy has been cancelled).
Because PI policies are claims-made policies, once they expire, they will not cover any more claims. That leaves you exposed years into the future after you retire or stop trading for mistakes you made in the past, which is why you should consider run-off cover.