That is a jeweler's saw. For anyone planning on using one without breaking lots of blades, the blades need to be pre-tensioned. Clamp blade in one end, put the handle in your belly and the other end of the saw against the bench (blade up). Push against the bench with your body and tighten the other blade clamp so the blade is tensioned.
What an insanely weird method.
Here's the right way:
Loosen the three thumbscrews (the bow length adjustment screw needs to be FULLY loosened so that the bow is able to move around in both length and be free to pivot a few degrees).
Clamp the blade sticking straight from the bow end.
Adjust the bow length until the blade sits in the right position in the handle clamp, and clamp it sticking straight out (there must be a little curve to the blade at this point because the two clamps are not pointed in the same direction!).
Grasp the handle end of the frame under the fingers of one hand, and push the rounded button on the end of the bow firmly with the thumb to apply initial tension.
Tighten the final thumbscrew. This is where the magic happens!
If you clamped both ends right, the blade should now exit the clamps without bending (if it's kinked, you should start over and get everything straight, or expect to break blades quickly).
The magic? The bow not only slides in and out, but also pivots a little. When you tighten the thumbscrew it pivots the bow into the final correct position (where the clamps now line up), and sets the final tension.
Oh, and that wingnut at the far end of your blade: completely unnecessary.
As for working vertically, that's not necessary if you have the right hand control. I like to work with my work clamped in a panavise, and adjust the angle to suit what I'm cutting.