Gonna poke my head in here as a Neuroscientist and say that there’s probably nothing magical about the music, (i.e. if it induces theta waves, etc.) because the science on this has always been relatively soft (mostly correlational data).
On the website, they specifically say “Pending Scientific Publication” with regards to “Results in 15 minutes” but I want to caution people that try this for 15 minutes and suddenly feel more focused: if you do anything with un-interrupted concentration for 15 minutes, it’s because you’ve settled into your task (known as Attentional Shift ). The brain needs a bit of time to “get into the groove”, and if you’ve ever meditated or done similar contemplative work, you can attest to feeling like longer sessions are better because it takes your brain a while to stop chattering and fussing about before giving complete focus to something.
I’ve personally tried Focus@Will and didn’t feel anything different than when I used free sounds on the Rain Rain app, or anything else that serves as background noise while I’m working (lately a lot of The Mercury Program and Pelican on Apple Music).