I think what Ray means is change your beats per measure to 1/2 that of the previous measure. You will do this where he points you to.

This is relevant to 4/4 time. For example, if your time is 4/4 set the 1/2 measure to 2/4 time. If your song is in 3/4 or another time it becomes more tricky. You may have to set up your changes over multiple bars.

In doing so you will want to change the next measure back to original beats. If this means you are cycling to the first measure then be sure to set the number of beats. BIAB will basically ignore this on the first chorus but will acknowledge when you return to that measure.

HTH

Happy Musiking!
Dennis