At the risk of sounding ignornant, I have always thought that electrolytes administered into a horses feed was a salty/chalky powder that made them incredibly thirsty. I know I would be awfully thirsty after eating something like that. I am also aware that they replace the ions, salts, etc etc in the horses body that are lost through sweat. However I was recently told that when electrolytes hit the horses intestines it causes a draw of water to the area, making for better digestion and thus making the horse thirsty. Never heard that one before.
Electrolytes ARE the salts and ions that all living things need for proper cellular function. Yes, they can be lost by sweat, by breathing, by normal excretion and a new and sufficient supply is always necessary. Normally this is taken care of by normal dietary intake (provided the diet is not deficient) but in cases where excessive losses of electrolytes are taking place (intense exercise, fever, etc.) then repletion/replacement is generally a good idea.
Yes, if we ingest too much sodium, this strongly triggers our thirst mechanism because it changes the concentration (osmolarity) of our blood slightly and our bodies are on a very highly tuned balance that has to be maintained. So we thirst, we drink, and the correction is made. Again assuming all systems are normal. Other minerals do not directly effect the thirst mechanism. Sodium is the big one, for general purposes.
Giving a healthy animal a sensible dose of electrolytes should not significantly cause fluid to be secreted into the intestines. Not enough to make a huge difference, anyway. But it is, in fact, how "osmotic diarrhea" happens when we use a product like milk of magnesia, so overdoing it can certainly cause a fluid shift. Which, again, a healthy body will quickly compensate for!!
Whether this causes "better digestion" (or is even happening on a meaningful level with normal doses of electrolytes) is highly doubtful.
Basically it always seems to me like a lot of people want to make physiology too complicated. (not the OP, but sellers of supplements, well meaning "experts", and people that incessantly insist on tinkering with their horses' digestion and fret over every microgram of every mineral under the sun)
A healthy animal, given the opportunity to ingest appropriate nutrients, will handle any excess without the slightest fuss, and if a deficiency is present has layers and layers of ways to compensate for this in the short term. No human meddling required.