What's the deal with -boundscheck?

[Meta via Digitalmars-d]

On Monday, 6 April 2015 at 05:15:43 UTC, Walter Bright wrote:
> I read the code to figure out what was happening.
>
> At some point, -release was changed so that bounds checking was 
> turned off for all but @safe code. A new switch was added, 
> -boundscheck=[on|safeonly|off]. It took me a while to find it 
> at:
>
> http://dlang.org/dmd-linux.html
>
> because whoever added it didn't realize that the list was 
> alphabetized and stuck it in there after -map.
>
> I don't know why the change was made.

I thought this was how bounds checking always worked. On in debug 
mode, off in everything except @safe code in release mode. I 
thought -release implied no contract checking, no asserts, AND no 
bounds checking.