It's short in the sense that you're selling before you buy and profiting off the thing bought/sold (bitcoin) going down between the two trades. Sell -> wait -> buy as opposed to the 'long' position of buy -> wait -> sell.

Since nobody-ish lets you borrow for bitcoin margin trades you have to "borrow from yourself" (already own bitcoin) and be long-bitcoin in the long run for that to make any sense.

But if you're a to-the-moon true believer that also thinks that bitcoin is in a bubble and going to make a temporary correction you could make a ton of bitcoin by being right... if you can actually make the trades and not get screwed by a counterparty.