A bond's cash flows are set in stone. If investors decide they need a higher return, the only free variable is what they are willing to pay, so the price falls until the fixed payments deliver the new yield. Yields up, prices down, every time.
This is the first thing to internalise before trading rates. A headline saying bonds sold off means yields rose. A flight to safety means bond buying, higher prices, lower yields. treasury-futures follow the price, so the futures contract falls when yields rise.
Example: a 10-year bond priced at par yielding 4%. Yields rise to 4.5%. With modified-duration near 8.1, the price falls roughly 8.1 x 0.5 = 4.05%, to about 95.95.
Related: yield-to-maturity, modified-duration, convexity, treasury-futures