this is going to be a weird explanation, and someone can explain better...but here it goes

when you car is at idle, the flywheel is spinning and the engine is running, you have just enough fuel going to keep the engine going...does the flywheel spin at idle? no matter, enough fuel to keep the engine running

if you think about it, when the clutch is out, its only powering the engine, then when you let go of the clutch, it starts pulling the car, and thus it needs more fuel to spin the wheels and transmission and such, along with pulling the weight of the car

so when you let off the clutch and don't give it gas, it puts too much stress on the engine and it can't stay alive, think about if you pulling say...a gallon of milk with a rubberband, you have enough strength to pull the milk and thats about it, so think about if you, as an exaggeration, add pulling a car to the weight of the milk, and adding gas is like adding more rubberbands, if you try and pull it all with one, the rubberband will snap, but if you add more then you can pull it without the rubberband snapping, but obviously your engine doesn't snap like a rubberband...jesus christ this is the worst explanation ever, i'm terrybly sorry but i hope you get the point