There are a number of ways to do gem baiting.
For instance, if @pyrhic believe I had just enough gems for a single battle and no way to resupply, he could marshal out with just enough troops to have me waste them on a small force, and leave me with not enough in the main fight. Alternatively, he could break siege this turn with a small force, having the AI use up all the gems, and then, after the break siege attempt fails, and I have elected to storm the castle, I would be doing it without most of my gems for the upcoming battle.
Alternatively, if I believed that @pyrhic had only a few gems, I might attempt to storm the castle with only a few troops, having him waste his few precious gems, and then strike again the next turn after he had used up most of his gems.
That might not be considered on the same scale though as the classic gem bait - cloud trapeze into a large force, and force them to use their gems before the much larger battle.
In any case, Illwinter has made gem baiting much harder. You used to be able to do it with a scout in Dominions 3. Imagine, a giant army was on your border, loaded to bear, and you attack with a scout. And then your enemy casts, Howl, Swarm, Fire Resistance, Mistform, Flight, Foul Vapors. Just everything set up for a giant killer fight, and use up dozens of gems against a scout. Illwinter now has the AI compare relative strength before allowing the AI use gems, especially if you toggle “Converse Gems” in the actions. But, if it comes to just mages, or small forces, you can usually trigger the gems to be used if your leader has enough items equiped.
Again, this is all about intentional gem baiting. It came up as I was deciding whether to attack the fort or not.