So BTC goes up 10% --> Alts go up 20%
BTC goes down 10% --> Alts go down 20%
This is true but as they said that experience is the best explanation, if you haven't experienced anyone before you might not know the true meaning of this because there is more to what he said to you.
There is Bitcoin season and there is altcoins season, Bitcoin market can pump for 2 weeks straight and it's possible that altcoins will remain stagnant and few will pump during that time, it's like that because everyone will be focus on the bitcoin market but as soon as the Bitcoin market is done, it will might do some corrections and then remain between some price but the fluctuations will not be that significant. This is when altcoins pickup and there moves is usually a fast and furious one, a coin can pump for 2 weeks straight and will not stop with 1000% and you will think it's stopping only for them to continue until market is stop by some news where it might do a heavy fall or correction for good coins, meme usually collapse after this kind of pumps.
I wish there was something that behaved conversely to the BTC price.
The market don't crash unless there is something that happened but because of the volatility nature of Bitcoin, they fall harder than bitcoin. When Bitcoin was having a small market and liquidity, it use to do what altcoins are doing today but with time it grow to become more stabilize due to the liquidity surrounding it. It's either you stick to Bitcoin, altcoins or hold stablecoins, it's as simple as that.