The bow in question is this one that the girl is using.
The setting is steampunk. The idea behind it is that the common enemy of the world are Zombies that have skin that is tougher, especially in the heart, which is the area they need to aim in order to kill them without them standing back up.
Since they can't penetrate the hearts, they develop these steam weapons, devised to do exactly that, basically to add more power to each bullet it is fired, and in the case of this bow, to add more power/speed to each flying arrow.
There's more to it but this is the TL;DR of it, and trust me, if you want to question it more the world will start falling apart, so don't even bother.
The questions here are:
- Is this bow possible?
- If so, would it work as it was intended, adding more power/speed to each arrow?
- If so, how do you think it would work?
- If it is a feasible bow, ergo, it can work by real world physics, what would be its pros and cons? would it be better than a normal bow?
- What would you need to keep in mind in order to safely use this bow without harming yourself?
Just found this question in another site, but I think this is the site were a more proper answer can be given and I was interested in it myself, mostly for the rule of cool than anything else, and may add something like this in my world if the answers prove positive and make my imagination run wild :)
Hope you have fun answering these questions too!