1.Why should i be varying the rpm of a hovercraft motors.

2.How does technically a electronic speed controller work?

can anybody provide some info on this.i will be thankful if anybody help me on this topic.

The ESC works like a carburetor in fuel engines.  In fuel engines, the carb controls the amount of fuel going into the engine, which in turn determines the speed of the engine.  Similarly, the ESC controls the amount of battery power that is delivered to the motors, thereby determining the speed of the motor. 

There are more aspects to this, like why brushless motors absolutely must have an ESC (timing etc).