OK I understand the basics, but can someone explain code copied from Greg's answer here:

a[1::-1]   # the first two items, reversed
a[:-3:-1]  # the last two items
a[-3::-1]  # everything except the last two items, reversed

To me the first one reads: slice from 2nd position to end, then reverse it.

The second one is slice from beginning to the -2 position then reverse it.

The third one is slice from -3rd position to end then reverse it.

Obviously I am wrong since they work as suggested, but can you please tell me why?

This will probably be deleted but here goes: First one is start at second position, head left as far as you can go. Second is start at rightmost, head left ending before -3. Third is start at-3 and head all the way to left.