Good question which does not come up too often. Here is what the manual says about "timebase".

"With the "Timebase" option you can play any RealTracks at normal, half time, double time or triple time. Half time is used for fast tempo songs when a much slower tempo RealTrack has been chosen. Double time is used for slow tempo songs (e.g. ballads) when a much faster tempo RealTrack is chosen. This allows you, for example, in a ballad at a tempo of 70, to add a RealTracks Sax solo with a tempo of 140, and play it as a double time, which will match the ballad tempo of 70. All of your existing RealTracks can be used at three different tempos (normal, half-time, double time)."

As you see, this does not necessarily change the "feel" as one might expect with the term "half time". This is more a BIAB term for stretching or compressing the available audio real tracks to cover broader tempos.

Perhaps you need to extend the bars 2x e.g., instead of \c,c,d,f\ change to \c c\ d f\. This is another feature in BIAB .

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