How does Google Now (Now on Tap) search inside apps?

So Google has just released a new feature for Google Now called Now on Tap. It enables a user to get contextual help from different apps (The Verge video example).

Does it simply take a screenshot of the current app and use OCR to extract the text?
Does it have "special Google permission" which allows it to read the values within the views?

What is Google Now on tap?

What to Know. Press and hold the Home button. A message will pop up > tap Turn On. To use the feature, tap the Home button or say, "OK Google" and ask a question. Use Google Now on Tap to get information about music, movies, or details about a restaurant.

Why does Google Search bring up my apps?

The 'Apps' permission allows Google Search to find the apps installed on your device. If you've got one too many apps, Google Search is an easy way to find and open them. What is this? Google Search is similar to Spotlight search but it can't search through all apps on your phone.

Primarily, it should work using the assist API added to the M Developer Preview. However as "Now on Tap" is proprietary and unreleased, we can only really guess at this time.