The Power Assurance Check is used to track the trend of an engine over time and requires you to apply power until the target Q, based on the current temp and PA, is achieved (OEI) . Usually you will be light on the skids because the target Q is in the region of 40-50% but you can also do this in flight (I don't recommend it though). The AFM describes the procedure clearly and also says that you should do a PAC daily. We do them every 25 hours.
The engine topping check requires you to fly OEI and pull power until a limit is reached (Q, N1, or ITT, whichever comes first). Should be done when an engine or engine component has been changed (governor, etc...). I always do them on contract when I take over a machine to ensure that, when I need it, full OEI power will be available.