Can someone show me the technic of downshifting in to accelerate?

Practice this the next time you are on the road;

Hold your speed steady, say 45 if you are on a secondary road or 55 if you are on the freeway.

Keep your foot exactly the same spot on the accelerator and depress the clutch. Slide the shifter into the next lower gear and release the clutch. Your downshift will be seamless and silky smooth.

Here's why;

The load the engine is under to keep the speed constant is released (obviously) when you depress the clutch. Because of this, the RPM's will rise a few hundred, almost always just enough to perfectly match the next lower gear.

You would have to slow the car down to repeat this procedure from 4th to 3rd, for instance, but the principle still stands.

What this does is teach you where the RPM levels are for a given speed and gear.

If I understand your question correctly, you want to know how to downshift in order to accelerate. Say you're driving at 50MPH, you are in 5th and you want to drop a gear to speed up faster. Just use the method I mentioned above, and you can do this smoothly as well.

The answer about over revving is no, probably not. 2300 is no where near the red line on most car engines and the RPM difference between the top gear and the next lower is not so much as to cause worry..

yes

depends what car/engine you have, pry not