If you are lucky to buy the right coin, enough quantities of it and sell it at the right moment.
While it is true one can get very high gainsfrom speculation on the altcoin market, it is too impredictable and I would not say it is reliable.

For example, there are projects that have serious people behind it and a proven track record of case use: Monero, Dash, Litecoin, etc. and still, these coins can be easily overcome in volume and capitalization by coins like Shiba Inu and Dogecoin, whichare meme or jokecoins. This leads people to focus on memecoins which fail most of the occasions.