If you want to earn in a bear market, you can use the bitcoin futures where you can bet on the downward movement of price as well on price bounce on resistance levels.
I think you mean shorting bitcoins which means selling the bitcoins at a higher price by borrowing them from the exchange at the current price and repaying back when the price has dropped further. It's not a bad strategy but usually when the market is down, the expectation is that the market will move upwards. For people who don't know what shorting is and how it works, I will give you a small example.
You had let's say $3k USDT in your exchange wallet
You borrow 0.1 BTC at the current market price, let's assume the current price is $30k so you got 0.1 BTC and you sold them at current market price.
You wait for the market to drop and let it drops to $25k
You buy back 0.1 BTC and pay the exchange back 0.1 BTC you borrowed.
So basically you sold @30k and paid them back @25k which means you made easy profits but again it's not that simple because the market can go up.