It's a k11 exploit, and you need one of those to install DSiWarehax, which in turn lets you downgrade.3dbrew said:svcWaitSynchronizationN does not decrement the references to valid handles in an array before returning an error when it encounters an invalid handle. This allows one to (slowly) overflow the reference count for a handle object to zero.
ARM11 kernel-mode code execution
An ARM11 kernel exploit can have a couple uses, but the primary one people are concerned about is the ability to do DSiWare hax with one 3DS.
As it stands, you need a pre-hacked 3DS to inject a save, then transfer it to another for hacking; such an exploit would allow you to do it without that.