The first thing that should be understood about jacking rifle rounds is that there could be many different causes. And there could be different causes for different model rifles. I am going to speak in general of toggle link rifles because that is what my experience is with.
It could be that the primer is not seated deep enough. Or that the shooter is pressing the trigger before the lever is fully closed. Or maybe they get out of rhythm and hit the trigger late and the lever is already opening. It could be some type of mechanical problem with the rifle that causes the shooter to get out of rhythm. The first step to correcting this issue is to figure out the cause.
I have seen a few people recommend correcting the issue by learning how to reload your rifle quickly. I am not discouraging this but it is not a fix for the root of the problem. Figure out the root cause and correct it.
If the primer is seated too shallow in the primer pocket, when the firing pin hits the primer, it moves deeper into the pocket and does allow a full strike which fails to cause detonation. It is important to check all your primers after loading your ammo. If you find one that is not seated properly, do not use it in a match. This is the best-case scenario because it is the easiest to correct.
If the shooter is pressing the trigger before the lever is closed, the trigger safety (assuming there is still one in the rifle) will not be disengaged yet and the trigger will not release the hammer. The shooter (in rhythm) would then open the lever to cycle the rifle without the hammer falling causing them to eject a live round. Fixing this would depend greatly on the shooters' technique while shooting their rifle. I tend to notice this more on shooters who keep their finger inside the trigger while cycling the rifle. What seemed to work for me was holding my finger farther forward in the trigger guard while cycling the rifle. This takes just a split second longer to get to the trigger and helped me not jack any more rounds.
If the shooter gets out of rhythm and hits the trigger late, there could be several causes for this. It could be a mechanical issue in the rifle. Or maybe they are pulling their finger out of the trigger guard during cycling and not getting it back in soon enough to press the trigger. The mechanical issue is self-explanatory. If you do not know how to correct it yourself, take your rifle to a qualified gunsmith. If the timing is late, try to keep your finger inside the trigger guard. This may require a loss of performance initially when getting used to it, but in the long run, could help to correct this issue. I’m not saying this technique is incorrect either. Missouri Lefty shoots his rifle this way for example. But if the technique is causing you to eject live rounds, it may not be for you.
There could be any number of other causes as well. The only way to be sure is to take videos of you running your rifle from your strong side when you eject a round. Then, analyze the video in slow motion and find the cause.
If you find it to be a problem of technique, then you can correct it with some dry fire practice. I do not recommend using full-weight dummy rounds to dry fire your rifle. They are much heavier than what the extractor is designed to handle and can cause premature wear on it. I use a dummy I loaded with no powder and a silicone-filled primer pocket. I then ground off one side of the rim and load that side up so that it does not extract but gives the firing pin something soft to strike when you pull the trigger. Start your reps very slowly and get your form perfect. Especially if you are training yourself to use a different technique with the trigger finger. Do 10 cycles of the rifle just like a 10-round dump, but slowly. If your timing is off during the dry fire, then go even slower until you can do several strings without any problems. A lot of times, you can figure out what is going on during these slow reps. Make sure you are pressing the trigger the same way you do during live fire. (i.e., slam fire, etc.). This may seem like overkill but the point is to train your muscle memory so that when you are shooting a string during a match there is no thought about it at all and your subconscious does all the work.
One thing I did as well included changing my stage prep at the Loading Table. I used to go over the rifle string in my mind by thinking about the multi-taps at a very fast pace. But in reality, I can not shoot my rifle as fast as I am visualizing shooting it. So when I went to shoot my rifle string, subconsciously, it was trying to shoot it at the pace I visualized causing me to be out of rhythm and to jack rounds. I now visualize my rifle strings at the pace I know I can run the rifle. My mind and body work together instead of against each other.
There could be many other causes as well, but in my experience, these are the three most common. I have had the alternate alias of Jack ‘Em Out James due to the number of live rounds I was once ejecting. It took me a lot of extra work and diligence to figure out the cause and correct it. But it is possible.
I hope this helps.