It is my understanding that spirits don't "jump on" anyone.
While most will likely not have heard of what I'm about to say, I've found experience to support it. Man is a trichotomy composed of body, soul, and spirit. Each contain various aspects of our being. Many might present it like this:
Body (Soma)
1.) Flesh
2.) Biological
3.) Earthly
Soul (Psuche)
1.) Reason
2.) Will
3.) Emotion
Spirit (Pneuma)
1.) Life Force (breath of life)
2.) Inner man (higher self)
3.) Intuition (spiritual awareness)
Most depict these parts of our trichotomy in a manner with the spirit residing in the center of man, surrounded by the soul, and the soul surrounded by the body. One chart I remember was presented by Clarence Larkin (I'll have it attached to this post at the bottom).
However, this is a little misleading. The "spirit" of a man, one's "life force" or "essence of life" or "breath of life", while seated at the core of a man's being... extends beyond the body as far as roughly 2 to 3 ft. (on average) and can actually "extend" even further. And so, we often "sense" other spirits or spiritual things that enter, or pass through, our field of "spirit". They are not "jumping on" us.... we are sensing them moving within "the spirit".
For example, it would be improper to say, "I felt a demon jump on me." It would be more accurate to say, "I feel something demonic in the spirit." However, the most reliable way to tell if an evil spirit is present is through the Holy Spirit's gift of "discerning spirits".
The discerning of spirits is valuable in that one with the gift can discern as to if they are dealing with a demonic spirit or a human spirit. The soul can set the tone or health of one's spirit. You see, sometimes people we encounter have a negativity from the soul that sets the general tone or health of their spirit. Imagine that a person stands next to you on a crowded bus. Do you "feel" their spirit? Yes. Why? Your spirits are "intermingling". And often when these people walk away they leave a residual negativity in your spirit. If their spirit is filled with demons, these demons might try to stick around in your spirit. Through prayer and being filled with the Holy Spirit we can cleanse the spirit of any negativity or demonic entities by driving them out with God's presence and strengthen our spirits from attack.
You can experiment with this reality by paying close attention when you come within a foot or two of someone. You might be surprised with what the Holy Spirit in you reveals just with them being so close. You might get visions, images, sense feelings, sensations, hear sounds or songs, etc. In fact, you'll discover that by extending your hand to shake hands, you're actually holding them at bay, preventing them from coming too close. Experiment with this, you'll be surprised at what you can pick up in another's spirit through the Spirit. So make sure that you're prayed up.
I'll also include a chart (with Larkin's) that I just made to try to describe what I'm trying to explain.
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