I have gone back as far as April 2009. I could find only one post, March 2010 about my thread. My interpretations of jockey instructions are thus.
1. Don't lead - means, stay near the front.

2. Miss break - means, go the the rear at start.

3. Don't go clear - means, win within the horses ability.

4. Don't trail - means, stay near the back.

5. Challenge later - means, horse lacks stamina or speed.

6. Challenge earlier - means, horse has both stamina and speed.
As I have only been playing the game for a few days, I suspect that my thoughts may be not quite 'kosher'. I would appreciate the thoughts of those of you 'what' know better.

All of the instructions listed below, I use with racing style instructions and haven't used them on their own but one would presume the horse will race at its favoured position in the field and abide by the instructions set by you pre-race (note: this doesn't always happen, especially when using inexperienced jockeys).

1. Dont Lead - I'd use this on a horse that either likes to race handy, and has a tendency to go off to quickly and not see out the trip. Ensuring that he doesn't lead will ensure he will be up with the pace, but won't be setting stupid fractions in the process and can try and pinch the race from 'in the van'. I'd use for this 'Race Handy, Don't Lead'.

2. Miss Break - This will mean your horse will lose a couple of lengths out the stalls, but if your horse is a front-runner, it'll mean he'll be rushed up to dispute things and probably hamper it's chances in the process. Personally with this approach, I'd only use it for a horse that likes being held up and your racing on a track with a big draw bias favouring the opposite side to where you are drawn. Thus allowing you to track over and in-behind a bunch of horses.

3. Don't Go Clear - This I use mostly for horses that I know are seriously well handicapped, although they still appear to win by several lengths which can be quite annoying if lining one up for a handicap coup along the way. It doesn't really serve much purpose for me but it'll do a job I suppose by not winning races by a distance, and so forth.

4. Don't Trail - Once again, I'd use this command with one of the race instructions, and I'd use this for something like a horse with a preferred racing style as 'restrained early' or 'even pace' to ensure they don't get far behind, most probably I'd use this with a Slight Hold Up command, ensuring that we don't get stuck too far back should their be a pedestrian gallop up front.

5. Challenge Later - This is one I would use if I'm going for a jump up in distance with a horse who isn't sure to stay. They will leave the challenge to the last possible minute (although sometimes its 2 furlongs out - lol), to ensure best possible placing.

6. Challenge Earlier - When you've got stamina assured and you've dropped back in trip by a furlong or so on the horses preffered distance, challenge earlier command is worth using as the jockey will get the horse rolling and into a stride for that powerful effort and more times than not, outstay it's rivals.

I hope this has been of assistance.

Thanks Duke, muchly appreciated. You use a few instructions that I have seen within the forum and within the game, used by my jockey. They are thus.

1. Race handy.
2. Held up.
3. Restrained early, seen this one a lot.
4. Even pace, as above.
5. Hold up.

Are these instructions 'hidden' because I can't find them in the game and I've looked everywhere I am able to access. I am assuming that #1. is the same as 'don't lead and held up.
3. Is the same as 'don't trail'.
4. Even pace has got me. I know what it means but can't find a connection to anything else.
5. Hold up. The same as don't lead.

Cause your wisdom to be forthcoming unto those in the darkness so that they also shall see the light, please Duke.

3. Restrained early will be 'slight hold up' which can be found from the dropdown menu of instructions to give to your jockey on the day of the race (the screen when clicking view race).

4. Even pace is suited for a horse that races mid-pack, so a horse that will race evenly with the pace and take up a position midway through the field.

5. Hold Up will be using the 'Held Up' instruction. I've supplied a screenshot for you below which gives you a list of commands you can choose to give your jockey.

Just click on the thumbnail and it will enlarge the picture in a new window.

And yea! Verily did the wise one speak and once more those in the darkness were again able to see the light. Thanks Duke.