Chart symbol:
The only difference between this stitch and Tunisian simple stitch (tss) is that the hook is inserted under the vertical bar from left to right, rather than right to left:
Getting the hook in the stitch this way is a bit tricky, so you may find it helpful to work it in two steps.
First, insert the hook in the stitch from right to left, as if you were to work a simple stitch, pull the bar and lift it slightly above the chain:
Remove the hook from the loop. It will be much easier now to insert it from left to right and work a twisted simple stitch:
Yarn over and pull up a loop:
Leave the loop on the hook:
Note, you do not go through the whole fabric when inserting your hook.