putting the explanation of why into words...

So the two key things to note are:

• The pattern (?<!X) (for any character X) matches at the beginning of the string (because there is no preceding character), and
• the double negation of (?<! (?! ) ) means that whatever the inner call to (?&lookback) returns (match/no match) is what the outer (?<lookback> ) will return. So what the last, innermost (furthest left) lookback returns is what the whole, outermost lookback will return.

So for the regex in question it boils down to two cases:

• If there is no preceding "a", then the regex will recurse all the way to the beginning of the string, where lookback will match.
• If there is a preceding "a", then (?<!a) will cause the match to fail.

Minor edit for clarification.