My opening scene drops into a sort of action sequence, but within that, uses a fair bit of subject-specific terms. To anyone familiar with the subject (horses/equestrian, with a mild western flare) these are basic, basic terms.... but I'm afraid to someone without this type of exposure might be confised, or worse, uninterested. I feel like these terms lend authenticity, give a clearer picture of the surroundings (a beginning to building my world), and I really feel like putting pause on the "action" to explain what, for instance, a halter is, and how it would be used, etc. I also feel like expaining to a less "educated" audience would insult and bore someone who knows more. I've been told both a) trust my audience. Giving credit, benifit of the doubt, etc, and b) this scene is too technical for non-horsey people. My instinct? Expect the reader to be mature: use context clues to figure out what's up. I've got a few more days before I can post for review, so looking for general opinions in the mean time.
You need to write to serve the story. If it's essential to the story that people know what a halter is, and if they can't figure it out from context, then you should explain what a halter is. Otherwise, leave it out. (And don't underestimate the power of context. If someone who knows nothing about horses reads "He slipped the halter over her head and attached the lead rope," they're going to know enough for most story purposes. It goes on the head, possibly a lead rope attaches to it, and I can figure out lead rope by just thinking about what those words mean - I'm good.)
I say write the story without bothering to explain the terminology. If the reader has to look something up, let them look something up. But as @BayView says, context is powerful, and intelligent readers will understand what you're saying. As I've said before around here somewhere, reading above your head is a great way to expand your vocabulary. Do NOT dumb down your stuff for the slowest, weakest reader! Make your readers catch up to you, and they'll be grateful down the road.
Option b) doesn't necessarily mean that you need to explain. You could instead arrange things so that the first mention of a new term has plenty of context, like @BayView's example.
All the Pretty Horses by Cormac McCarthy comes to mind for terms relating to horses. I know absolutely NOTHING about horses, but the way he wrote the parts that included specialized terms allowed me to gather a rough meaning via context. I lost nothing in terms of story from not having prior knowledge of those specific terms. Context really is king.
also if you do need to explain something very technical have one character explain it to another rather than explaining directly to the reader in omnipresent.
There are lots of ways to handle this. A very good one is to leave out the terminology, or to explain it in the story. Example. The coach tells the rider, "don't sit back on the cantle! You look like you're drivin' a truck!" And the rider does her best to scrunch up toward the front of the saddle, but slips back after just a few strides. Coach throws hat on ground and stomps away. Just make sure it means something to the story. Like showing how tough the coach is and how much the rider wants to improve, and why she wants to improve. To win over someone else, to be competent at something, to be a better pal to her horse, whatever. In the story you kinda painted yourself into a corner as the character wants nothing to do with the novices (no one to explain anything to), she's already experienced (she won't be learning any of the jargon herself) and hangs around only with the experienced people (they all know it too). Horses are particularly difficult in this respect because there is an immense body of jargon that most ah....'normal' people would never encounter, but even worse, because even different styles of riding have completely different sets of jargon. And because it IS unfair and lazy to make the reader run for the dictionary too often. What's too often? Three times.