HSQLDB SQL Implementation

In the Lucee docs under Query of Queries (QoQ) section there’s this heading:

HSQLDB SQL Implementation
This is the fallback for when Lucee’s SQL implementation can’t handle the QoQ syntax. See the HSQLDB documentation for details.

What does that (specifically) mean? How does one use that fallback?

I note that the std. runtime that comes with Lucee is 1.8.0 and that installing the Hypersonic SQL DB extension adds a 2.4.0 version. So does installing this extension mean the Lucee’s QoQ functionality now uses this new jar by default? Or is this extension unrelated to Lucee QoQ?

We are forced to use QoQ quite often as we create middleware between different applications with different database engines so any upgrade/advancement/etc. in speed or functionality would be worth looking into.