1. The VIX essentially is a measurement of expected volatility of 1 standard deviation over the forward 30 days.
2. You can't trade the VIX, but can trade VIX future contracts.
3. The VXX and UVXY are priced via VIX front and second month future contracts.
4. UVXY is essentially a 2x ETF of VXX.
5. The VXX is rebalanced daily to sell front month contracts to buy second month contracts. If there's 22 days in March, 1/22 is 4.5%. Therefore, 4.5% of front month contracts are sold to buy more second month contracts.
6. If future contract prices are higher than spot price, it's said to by in contango. Since, it's most always in contango, future prices are diverging towards the spot price. Therefore, it's always decreasing in value.
7. When there's a volatile event, front month future prices will rise and be higher than second month future prices. This is called backwardation. Essentially you're selling high and buying low. This causes the VXX and UVXY in raise in price until future prices adjust and contango sets back in and prices will continue to fall.
This is how I understand the VXX/UVXY. I could be wrong.