Cylinder Power Balance Test
A cylinder power balance test tests each cylinder's effectiveness compared to the others. A weak cylinder could be caused by an ignition, fuel delivery, or mechanical problems such as a faulty valve or worn piston rings. As this question suggests, there may be one or more cylinders involved. A faulty head gasket, cracked cylinder head or block will cause an rpm drop on adjacent cylinders.
For many years, removing a plug boot from its sparkplug was standard procedure. This is not always true today; a prolonged open in the secondary ignition system may cause damage to the coil or ignition module. Sometimes a sensor, like an O2 sensor, will need to be disconnected so the computer will not try to compensate for the sudden change in conditions. Check with the manufacturer's specifications before proceeding and remember safety first. Use a grabber or a tool that will protect your hand from any shock that may result from removing the plug cover.
Notice the rpm drop as power is removed from the cylinder. If a cylinder's drop is not consistent with the others, this indicates a problem with that cylinder. An engine analyzer tests each cylinder and then compares the results. It is easy to do the same on a piece of paper. An engine analyzer or dedicated tool is best for testing coil on plug COP ignition systems. This is because coil damage may result from removing a coil while the engine is running. Check with the manual; there are many ways to cancel a COP cylinder, such as disconnecting the associated connector.