Turning a draft will rotate the draft so that the threading becomes the treadling and vica-versa.
Suppose you wanted to weave the following drat using one shuttle, instead of alternating the two colors in the weft. Note that this draft using 10 treadles.
Clicking the Turn Draft Icon one time:
Yields the following:
Now the draft requires 10 shafts and uses 8 treadles. Look at the tie-up, it also has turned accordingly.