Call it the Masayoshi Son mathematical distortion field.

The founder of SoftBank Group Corp. added about 1.94 trillion ($18 billion) to the market value of his company Thursday — by unveiling a plan to buy back shares worth less than a third of that amount. Based on his own stake, Son’s net worth rose by about $5 billion.

The billionaire is no stranger to funny math. Yesterday, he opened his earnings presentation with a riddle: “25 – 4 = 9?” The formula is the 25 trillion yen value of SoftBank’s assets minus 4 trillion yen in debt, far from being equal to its market capitalization of 9 trillion yen. It was designed to capture Son’s longstanding argument that SoftBank’s share price doesn’t reflect the value of its business and investments, a gap he’s been trying to close for years.

Even after today’s boost, SoftBank’s total value is a long way from Son’s goal. Here’s what’s at play.

1. How big is the buyback?

It’s the biggest ever for SoftBank. The company plans to buy back as much as 600 billion yen starting today through the end of January next year, and the stock will be retired. SoftBank’s shares jumped by their daily limit, gaining 18 percent in Tokyo on Thursday, the most in a decade.

Son has had success with buybacks in the past. In 2016, he announced the Tokyo-based company would buy as much as 500 billion yen, which sent shares up by the limit the next day. The price doubled over the next year.

2. How is he paying for this?

With money from mom-and-pop investors in Japan. The buyback will be funded by the proceeds from the 2.4 trillion yen initial public offering of the company’s telecom unit in December, which was marketed to individuals. While SoftBank Group’s shares have gained 36 percent this year, the telecommunications unit is trading 13 percent below its IPO price of 1,500 yen.

Even so, it’s worth remembering that SoftBank’s shares retreated after they doubled following a share buyback in 2016.

3. How much is SoftBank Group worth?

According to Son, 21 trillion yen net of debt. That includes 12.5 trillion yen stake in Alibaba Group Holding Ltd., 2.4 trillion yen telecoms unit, 2.6 trillion yen U.S. carrier Sprint Corp., 2.7 trillion yen each in the Vision Fund and chipmaker Arm Holdings Plc., and 600 billion yen in Yahoo Japan Corp. SoftBank’s market cap was closing in on 11 trillion on Thursday.

SoftBank is also taking steps to cut debt, and said it’s spending 700 billion yen from the IPO proceeds to repay creditors.

The shares of SoftBank closed at 9,962 yen on Thursday, still a 50 percent discount to Son’s sum-of-the-parts calculation that puts shareholder value at 20,055 yen a share.

4. What could help close the gap?

Most of SoftBank’s assets are in mature companies like Alibaba, Sprint and Yahoo Japan. That’s why Son has been focusing more on the $100 billion Vision Fund and its portfolio of private companies that includes the world’s biggest ride-hailing company Uber Technologies Inc. and co-working giant WeWork Cos.

Thanks to valuation gains, profits from the Vision Fund and SoftBank’s own Delta Fund more than tripled to 176 billion yen in the quarter ended Dec. 31. The Vision Fund has emerged as a major contributor to earnings over the past year. While the returns can be difficult to predict, Son has promised several exits annually. This year, investors could look forward to a windfall from the planned listings of Slack Technologies Inc. and Uber this year.