I can just picture a shadow grabbing someone and pulling them into the bushes lol. A spell or curse will 'jump' its target the same way, you direct it at them. When you cast any type of spell you charge energy and through visualization direct it at the target. If it's a love spell you visualize your crush asking you out as you send your love and positive energy towards them. A curse is the opposite, you visualize your enemy getting hurt while you focus your anger and negative energy in their direction.
In Western magic, curse is usually a negative thing, but in some Asian culture, curse is neutral; it can be either/neither good or bad. In Japanese, the word for curse is noroi and the word for spell is usually called omajinai, but the way you write those two characters in kanji are the same for both noroi and omajinai. In other sense, every kind of spell is a form of curse because the practice is associated with strong emotions from the caster whether it's anger, love, sadness...etc. In onmyouji, there're no differences between a love spell and curse because they're both the same thing; you're "cursing" someone by casting your unwanted emotion to them without consultation against their will. You want to cast a love spell because you're trying to get the persons' attention without considering the persons' intention towards you. If the caster already knows that the person is in love with him/her, what's the point of casting the spell in the first place? Same goes with cursing a person, you want to destroy the person without them knowing it and not thinking too much about the consequence.
Personally from what I studied, there're no bad spells or good spells because magic behaves like technology, it can be used for evil or for good since ad hominem spells that's meant to manipulate person's thought or life are caused by the selfish desire of the caster without thinking too much about the victims of their spell.
You are all discussing how it is only fear that activates the curse as if it is a fact, but that is not the only element that triggers the affects. If the person performing the curse has enough knowledge and experience with what they are doing, then it is possible they are smart and strong enough (mentally speaking) to break through the "no-fear" zone of the victim or target, if that makes sense. And if they keep persisting, it adds up the energy and more misfortune can arise. However, it is also all on the view of the target: if they know what is happening and they accept something unusual is happening to them, it is true that they are going to more likely be aware of what is going on. They can also take this "bad experience" that has occurred and perceive it as something good, thus will only frustrate the caster and will likely leave it to faulter in the end.
If you are good when it comes to working with energy, then you should be able to detect what needs to be protected (whether it is an injury on your body or some other object being injured), and then you can cleanse that thing, and shield it.
Also, backfires are a personal belief. If they caster of the curse does not believe in karma or backfires, like myself, then they would not be affected in any way.