Is the Houdini Stop safe?  

 Yes. The Houdini Stop chest clip has been crash tested at the Autoliv Test Facility in Melbourne, Australia. To the AS/NZC 1754 Standard. In short the Houdini Stop was found not to interfere with the performance of any part of the car seat. In all tests the Houdini Stop did not move up and have any contact at all with the throat area.