Here's the underlying math on how it's done:

Given a number, say 5892, its "digital root"
is found by adding its digits: 5 + 8 + 9 + 2 = 24
If the sum has more than one digit, add its digits:
2 + 4 = 6. The digit root of 5892 is 6.

If we scramble the digits, the digital root
is still 6.

Subtracting the two numbers, the digit root
of the difference will be 0 or 9.
(There is a lengthy reason for this ...)

When we omit a digit in the difference,
it can be easily determined.

For example: 5892 - 2598 = 3294
Note that the digital root of 3294 is 9.

If we circle the "2" and enter "394", the
digital root of 394 is 7.
Since the digit root was originally 9,
they know that a "2" was omitted