&-(_:_:)

The overflow subtraction operator (&-) discards any bits that overflow the fixed width of the integer type. In the following example, the difference of 10 and 21 is less than zero, the minimum representable UInt value, so the result is the partial value after discarding the overflowing bits.

let x: UInt8 = 21 &- 10
// x == 11
let y: UInt8 = 10 &- 21
// y == 245 (after overflow)

For more about arithmetic with overflow operators, see Overflow Operators in The Swift Programming Language.