For example,
0) SoftBank loans extra cash to Masayoshi Son.
1) Masayoshi Son invests a portion in the next Vision fund which makes 40% returns, and the rest in safer strategies (public equities, long-term debt)
2) Masayoshi Son makes a killing, has no trouble repaying the loan.
3) SoftBank just grew their cash reserves well above the market rate.
The key assumption here is 2). But since this is a loan, and Masayoshi Son invested most of the money in safe investments (and has a lot of wealth anyways), even if the Vision fund underperforms, SoftBank can be confident they can still make money back (step 3). They're using their advantages to build their own unique loan product.