To start out, I have $2000 of savings. I buy the BTC for 800, so now I have $1200 + 1BTC.
I sell the BTC for $1000, so now I have $2200.
I buy the BTC again for $1100, so now I have $1100 + 1BTC. I sell the BTC for $1300. Now I have $2400, or:
$400 more than I did before.