Integral literals with Exp?

[John Colvin via Digitalmars-d]

On Monday, 15 September 2014 at 00:54:40 UTC, Walter Bright wrote:
> On 9/13/2014 12:23 AM, bearophile wrote:
>> Is it a good idea to accept code like this, to shorten some 
>> constants?
>>
>> void main() {
>>     int x = 1e6;
>> }
>
> 1_000_000 solves  that problem.

How does adding extra _s help with shortening constants?