The ETC1 format consists of two block colors, two intensity table selectors, two mode bits ("diff" and "flip"), and 16 2-bit selectors. Here are some simple visualizations of what this encoded data looks like.
The original image (kodim14):
The ETC1 encoded image (using rg_etc1 in slow mode - modified to use perceptual colorspace metrics):
Here's the selector image (the 2-bit selectors have been scaled up to 0-255):
Subblock 0's intensity, scaled from 0-7 to 0-255:
Subblock 1's color, expanded to 8,8,8:
Subblock 1's intensity, scaled from 0-7 to 0-255:
The "flip" mode bits (white=flipped):