I had a comment asking me how did I short ETH (20x btw).
Basically, I used the Binance futures market on cross collateralized setting and went short ETH and long BTC for the same amount of money at the same time.
This way if ETH is weaker than BTC, the sum will be positive. Even if both are falling, if ETH is falling faster, the short position will bring in more profits than the long position on BTC is losing.
Right now, ETH/BTC pair is down and even though both are falling, my loss on BTC long position is 7% while my gain on ETH short position is 14%.
I hope that clears this up.
When the product you need is not available, sometimes you can make it from scratch by using cross settings.
Generally you should use isolated.