Its simple math. Lets talk about a 2x etf
IF, the price of the index is $100, and it goes down10%, its now worth $90
THEN, The 2x ETF will go from $100 to $80
IF, on day 2, the index goes up 10% again, the index goes up to $99
Then, the 2x goes up 20%, and the index is now at $96
Now...