Note: Your gearing (angle of chain) and the size of the phantom chainring can really affect the performance of the setup - a phantom chainring can greatly reduce the number of engaged teeth on the cog. As someone else said, it makes the chain skip over the teeth. Probably no worse than running a tensioner in "push-down" mode, but I've seen skipping w/ that setup as well, which is why people like to run "push-up" - more engagement. But I digress.
Also, I second the comment about it being more appropriate for SS than FG.