A Short Call is best used when you expect the underlying asset to fall moderately. It would still benefit if the underlying asset remains at the same level, because the time decay factor will always be in your favour as the time value of Call option will reduce over a period of time as you reach near to expiry. This is a good strategy to use because it gives you upfront credit, which will help you to somewhat offset the margin. But by initiating this position you are exposed to potentially unlimited losses if underlying assets goes dramatically high in price.