I do not critique very often, so when i do, I hope to be helpful, insightful, and come across constructive. Please, do not hesitate to be honest.
I would like to start off by saying, that this image has a good start to it. It has a good motion to it, and if finished well, has good potential in catching a judges eye.
Vision - the lines are the not the easiest on the eyes, but they do seem complete. I would suggest using a thinner brush (lines) for making linearts, as they have a smoother look to them. They are easier on the viewer. These thick lines look rough, but that is not always a bad thing.
Originality - I must say, in all the sliding stop images ive seen, aside from one i can think of that i did for Jax, this has to be one of the most aggressive stops ive seen in a while. it is a fresh sight for sore eyes, per se. The angle is also a good one. It compliments the pose well.
Technique - It seems to me that your technique is improving! Your shading is getting more detailed and it seems you have been doing some studying. On the side, it does seem to look a little blurry, which can also be hard on the viewer if they are sensitive to that kind of thing. You have paid attention to the smaller details in you line, like the buckles and straps on the over-reach boots, etc. Keep up the good work!
Impact - The overall impact that this image had on me was a rush. I could feel the pull of the gravity, the adrenaline of riding a good stop. The excitement and power is evident in your posing, as well as on your riders face. It was an overall joy to look at.
Overall, this was a pleasant image to view. I hope it will look just as complete when it is finished and that im not the only one who enjoyed the thrill of the slide. With the right touches i think it would make an excellent show entry.