Using an example will help.
For instance let a = 6 and b = 13.
a = a + b
Now a = 19 and b is unchanged at 13.
b - a - b or b = 19 - 13
Now a = 19 (unchanged) and b = 6.
a = a - b or a = 19 - 6
Now a = 13 and b = 6. The values are swapped.
For pure functionality, I don't recommend this method because it only works with summable values. Swapping using a temporary variable works in all cases.