Shorting the Nasdaq 100 involves taking a position that the index will fall. This can be done by selling short a Nasdaq 100 contract or shorting constituent stocks. Alternatively, traders may be interested in shorting a Nasdaq 100 ETF. For options, you can buy Put options on Nasdaq 100 stocks if you believe them to be overvalued, or buy a Put option on a Nasdaq 100 ETF.
Read more about shorting a market.