LMD Flatten works just like Statamic's built-in flatten modifier except you can pass it a depth
to flatten.
This isn't possible with Statamic core, because it runs on Laravel 5.1 (as of v2.1), and the feature debuted in Laravel 5.2.
{{# Just like core `flatten`. #}}
{{ array_value | lmd_flatten }}
{{# Only flatten one level. #}}
{{ array_value | lmd_flatten:1 }}
The underlying code changed a bit in 5.3, but it's not out yet, so for pretty docs, check out Laravel 5.2's Docs.
If you're paranoid something may be different, you can also see the 5.3 docs-in-progress.