Just to clarify we're talking about the same thing. Passthrough (also called bit-streaming) is sending compressed Dolby/DTS audio over HDMI. Your receiver then converts it to LPCM before using its internal DAC to turn it into sound your ears can hear. We can do this, but only for 16-bit formats.
With Infuse set to Auto, it will first decompress the Dolby/DTS audio, and send this decompressed audio to your receiver as LPCM. The same characteristics apply (quality level, channel counts, volume levels, etc...) and your receiver will then use its internal DAC to turn it into something your ears can hear. This method was recommended by DTS as the best option for getting 24-bit audio from the Apple TV to your receiver.
Hope this helps! 🙂