Trading in Chinese stocks was suspended Thursday for a second day this week after a dramatic plunge that sent shocks through global markets.

Dealing was briefly halted after the CSI 300 stock index fell 5%. When markets re-opened, losses reached 7% within seconds, triggering a complete suspension for the day. The shortened trading session lasted just 30 minutes.The abrupt decline triggered so-called circuit breakers, which Chinese authorities recently implemented in a bid to tame the country's volatile markets. The new circuit breakers were also used to halt trading early on Monday.

While circuit breakers initially limit losses, they may encourage investors to sell more. Why? Observers say the first breaker is reached too quickly, and investors use the cool-down period to line up additional sell orders.

The CSI 300, which tracks stocks in Shanghai and Shenzhen, has already fallen 12% in 2016.

China's central bank responded Thursday by announcing it would pump $10.6 billion into the financial system. That follows an injection of $20 billion on Tuesday.

The moves are designed to juice stocks and calm mainland markets, which are dominated by small savers who put more faith in speculative investing newsletters than company fundamentals. But observers say they also signal that China's leaders are concerned about the economy.

"Investors recognize that the [central bank's] actions serve as confirmation that China's economy is slowing in a meaningful fashion, which has real repercussions on global ... growth," Mike O'Rourke, chief market strategist at JonesTrading, wrote in a note.Two separate reports this week fanned fears of slower growth in the world's second-largest economy. One showed that China's services sector grew at the weakest pace in 17 months in December -- another revealed that activity had slowed in the country's key factory sector.

Another concern is China's weakening currency: Before trading began Thursday, China's central bank set the yuan's value at its weakest level since March 2011. A weaker currency can help Chinese exporters and support growth, but it can also push money out of the country and hurt asset values.

Worries over the currency have intensified since a surprise devaluation in August. At the time, Beijing said it was hoping to allow market forces more control over the yuan -- but the central bank has spent billions in recent months to prop up the currency. On Thursday, regulators set the yuan's daily trading limit sharply lower, the largest such decline since August.

The yuan has lost nearly 6% against the U.S. dollar since August.

The pain extended beyond mainland China on Thursday. Hong Kong's Hang Seng dropped 3.1%, while Japan's Nikkei shed 2.3%. U.S. stock futures were sharply lower.

Concerns about China have also helped ravage oil prices -- a trend that in turn hurts global economies and further unsettles stocks. Oil is now trading below $33 a barrel.

Most investing professionals recently surveyed by CNNMoney listed China as the biggest risk to U.S. stocks. When Chinese markets were halted Monday, the move triggered a global selloff, including losses of roughly 2% in the U.S.