The flyer holds the bar with their hands shoulder-width apart and one hand reversed. The hands are one one side of the bar, with the reversed hand in the centre. To turn to the left, the right hand is reversed (shown in the photo). To turn to the right, the left hand is reversed.
This grip is used when the flyer needs to make a position on the bar before turning around, for example for a Planche Turnaround.
Note that the image shows the flyer with their thumbs under the bar (around). This is the most common way, however some schools prefer the thumbs to be over the bar.
You can find out more about this trick in my books:
IMAGE CREDIT: Diana