alt Hacker NewsIt doesn’t signal anything. Your heart rate drops because you’re inhaling less oxygen so your heart is like “we don’t need this high fi flow” and slows down blood flow to lower energy expenditure.
Lower energy state always wins unless chasing energy source.
Note that fast breathing doesn’t necessarily mean high oxygen uptake. Deep breaths result in higher oxygen uptake (since you spend less time just moving the same stale air up and down your airways), and deep breaths are usually easier to perform when breathing slowly.