Fractional Kelly means staking a set fraction of the full Kelly amount, such as one half or one quarter, to reduce swings.
Real edges are estimated with error and full Kelly produces violent drawdowns, so a fraction sharply cuts volatility of returns. This is why practical risk rules like the 1 percent rule sit far below full Kelly.
Formula / example: Common choices are 0.5 (half Kelly) or 0.25 (quarter Kelly)