Siri sends back whatever is necessary for it to translate, compute, and send back the response.
In the case of Open Settings, your voice data for the session "Open Settings" will be sent back to Apple along with your IP address and list of installed applications, so it can translate open settings on it's end, formulate the proper URI for settings. (e.g. settings:\\dosomething), then it sends that URI back to your device using the IP address you supplied so it can execute that URI on your device.
Apple may capture some of your data in the form of a hash which will be made on your phone. This hash will be sent up to Apple for intuitive suggestions for you. It's not like they send everything up to their servers, just bits and pieces.
The quick
the lazy dog
the dog
quick fox
the brown
jumps over
brown fox
over the
quick brown
fox jumps
the fox
.
If you had an idea of what I wrote, it's because you are familiar with the sentence "The quick brown fox jumps over the lazy dog." (used to simulate every letter of the alphabet in fonts)
It's a common phrase, but as you can see if it were anything else, you'd have no idea what I typed. Especially if it were encrypted from a hexadecimal format with some strong encryption.
The fact is the data isn't easy to understand unencrypted, harder to break encrypted, and this still isn't the last form, it's probably uploaded after being compressed into a binary tree. Which means you are streaming about 100 1s and 0s for what would be this post.
Microsoft screwed up because they didn't mention anything about anonymizing your data, they just say we collect it.