[bridsanaeds]

So if i understand correctly can double clutching when downshifting prevent my car from Feeling as if something hit the engine when i downshit to a lower gear at high speed through straight shift.

That's exactly what it can do. Though you don't really have to double clutch to alleviate most of that, rev-matching alone can make all the difference.

if this is the case then i can do it like for example (3rd gear ----> cluch ----> netural ------> cluch ------> 2nd Gear) ???

Exactly. An example where its effect is even more obvious would be shifting from 4th through to 2nd.

so if this the case then is there any benfit for this when upshifting???

Not much really, the only advantage to it in a synchro-mesh box is that the revs will drop a little quicker. Say you've just revved the guts out of 2nd to get onto the highway, then want to drop into top gear, a double-clutch upshift would help make things a little smoother, but just leaving the clutch in for a while longer would have pretty much the same effect.

also when using double cluch method i described above what could go wrong in the car????

You'll know if you're doing something wrong. The only way you'll damage the car is by doing it wrong, but you run that risk with normal shifting anyway. Things like mis-timing the clutch pedal could mean you grind the gears. Maybe if you're in neutral and get the throttle wrong you could over-rev the engine

If you get good at it and do it all the time, you'll actually put less wear on the cars components. Although you'll be pushing the clutch pedal twice as much (ok, maybe you'd wear out the pedal hinge quicker or something, no big deal), the clutch itself will spend less time slipping, meaning it won't wear as quickly. Smoothing everything out also means there's less general wear on the gears and drive train. It also preserves the lifespan of the synchro-mesh because it's getting far less use.