I pull the plunger back just enough to break the seal and give some room for some blood to flow in before it pierces the skin. Usually 1/4 to 1/2 inch. Then I push it in where I think the vein is at. If I miss, I don't usually pull it completely out, just move it around to find the vein.
One of my problems when I started was pushing all the way through the vein. In a lot of animals it is barely under the skin so your needle is running parallel to the tail if you are holding the tail straight up. The little three ml syringes don't take much to pull back and I have learned to hold it with my thumb and first two fingers and pull back the syringe with the ring and pinky finger while the other hand holds the tail.
I really wouldn't think this way is better, it just allows a little more room for error when leaning how to do it. If you miss, push the plunger back to the starting point and try again.
One thing I have learned is that it easier to do with just a head gate and chute if you can stand on the edge of the chute to elevate yourself somewhat. It makes it much easier on the back to hold the tail with one hand and bend over the top rail to access the back of the cow. You also don't get $hyte on so much.
Another is that it is not 100% accurate. I had a nice young second calf cow that tested open but luckily I liked her enough to move her with the bull. She calved about 7 months later.
In one group I did last year, the technician said that one cow was a "weak" pregnancy. I didn't know what that was so I emailed him. He said in a lot of cases, its when Ia cow aborts a very young fetus shortly before you tested her. Sure enough he was right as it took about 10 months after that for her to calve. Its a good management tool that doesn't cost much.
