To fully understand how brushed and brushless ESCs work, it is necessary to understand the concept of pulse-width modulation.
First, imagine that you have a water pipe with a valve which can only be fully opened or fully closed. If you open this valve, the water flows through the pipe at 10 gallons per minute. If you close this valve, it stops the water flow.
Now, if you want a water flow of 5 gallons per minute, you can open the valve for 5 seconds, then close it for 5 seconds, and repeat. Since the valve is open for 50% of the time, the average water flow will be half of the max flow rate, or 5 gallons per minute.
If you want a water flow of 2 gallons per minute, you can open the valve for 2 seconds, then close the valve for 8 seconds. This will give you an average water flow of 2 gallons per minute.
An ESC works in the exactly the same way. If the throttle signal is 50%, then the ESC will apply full power to the motor for 50% of the time. If the throttle input is 20%, then the ESC will apply full power to the motor for 20% of the time.
This pulse-width modulation technique has several important limitation. If you ignore these limitations, you will overload your power systems, and your heli will likely crash and/or the motor, ESC, or battery may be damaged.
You must select an ESC that can handle the current draw at full throttle, even if you do not plan to fly the helicopter at full throttle.
For example, if a Corona hovers at half throttle and draws 14 amps, then the current draw from the battery and ESC is NOT 14 amps. What actually happens is the battery and ESC are delivering 28 amps to the motor only 50% of the time.
Therefore, if you use a Phoenix 25 for the Corona, it will be overloaded even when hovering, and will probably overheat and shut down when you apply more throttle.
Also, you want to select a motor that will hover your helicopter at no lower than 50% throttle for this reason.
If you select a motor with the proper Kv and the CP helicopter reaches 1600 rpm of headspeed at 90% throttle and draws an average of 18 amps of current, then the ESC and battery are actually supplying 20 amps of current for 90% of the time.
If you select a motor with an excessively high Kv rating and the CP helicopter reaches 1600 rpm of headspeed at 25% throttle and draws an average of 18 amps of current, then the ESC and battery are actually supplying 72 amps of current for 25% of the time!
Therefore, if you use a motor with a very high Kv rating and you are forced to use a low throttle setting to compensate for the high Kv, this will increase the load on the ESC and battery. In extreme cases, this will shorten the life or destroy the ESC and/or battery.