Ok, so here is my best shot, Meli. I think you, and you're not alone in this, have a backwards perspective on spasticity. Spasticity is an external symptom of a particular kind of dysfunction in the spine.
Our spinal cords are the main wiring systems of the body, just like the electric wires strung along telephone poles outside your house are for your neighborhood. Now if your lights in your kitchen all turn off, that's a symptom of a problem. You can instantly perceive there is no light. But the cause of that no light isn't immediately obvious: did you blow a fuse? did your lightbulb die? did the wire to your house get cut by a storm? did you forget to pay your electric bill? or is your neighborhood out of power?
There is no amount of staring at the now-dark ceiling that will tell you that. Instead, you survey the situation. If most of your house has power, but your kitchen doesn't, you probably blew a fuse. If just the lightbulb is out, but the microwave is working, you need a new microwave. If your house is out, but your neighbors have power, you should be looking at your electric bill payments and the wire to your house. If no one in a three block radius has power, we can safely say that your house being out is part of a greater black out.
Tightness, stiffness, jerkiness: those are all symptoms, but they are as unspecific as your kitchen light going out. Your kitchen light will look just as dark no matter why it doesn't have power. Stiffness can be a joint problem, a muscle problem, a neurological problem, or a wearing overly tight gloves problem. Clinical tests are the survey that narrows down the cause of the symptoms.
Back to spasticity:
When you initiate a movement by thinking, two commands are sent out and down the spine. So if I do a bicep curl, a "contract" message is sent to my bicep, making it shorter, and pulling my arm up. For that to suceed, a complimentary "relax" message has to be sent to my tricep, telling it to become longer and looser. This basic contract-relax rythym is how every motor function in your body works.
Spasticity is a spinal problem, a problem in the wiring. When the upper motor neurons are damaged, they don't send those relax/contract messages properly, or in our power analogy: they don't power the bicep and turn the tricep off. Instead, a lot of unfiltered "sparks" of errant contract message/power are sent into the tricep. If your wiring isn't too busted, much more power will be sent to the bicep, so it'll win in the end, the limb moves, but stiffly or jerkily. That stiffness or jerky movement is what we get to see visibly of the poorly working invisible wiring in the spine.
That is why spasticity is measured by passive movement. Passive movement of a relaxed or paralyzed limb should be smooth. If there are all these sparks of contract/go juice, it isn't smooth, the tester has to overcome that flow of power into the muscle that should be relaxing. The faster he tries to move the limb, the harder it will resist. The problem the patient encounters isn't that velocity-dependent resistance itself, but rather the symptoms of the bad wiring it points to.
So all people who have spasticity share this spinal wiring breakdown. But how that looks and feels when we try to do stuff varies. For those of us with upper motor neuron diseases, our UMNs are dying a death by a thousand cuts. Our wires are frayed, insulation missing in many places. Like frayed wiring, our power signals are inconsistent, which means that how stiff we are can vary due to posture or external factors that change spinal position. (Think of tweaking the wire position of a poorly working computer mouse.) Where those thousand cuts are, how much signal they are blocking vs not blocking, no two of us would describe the end result the same way. So we can't tell you how spasticity "feels"... that is very dependent on our individual levels and locations of UMN damage. We aren't in the same boat because of how it feels, but because of the underlying damage to our spines that we share.
Contrast that with a bullet through the spine. If it leaves a small amount of motor pathway intact, that paraplegic may also have spasticity. For most of them, they limbs they have spasticity in are completely paralyzed, incapable of being used intentionally. But the same "go juice" spine malfunction we have they can share. They have no normal use of the limb, no proper signal at all, so spasticity in their life looks like sudden jittery rythmic motions of their legs or feet that knocks their feet off of wheelchair footplates, makes it hard to cath or put their socks on, etc. Bad spasticity for them can knock their whole wheelchair over backwards. In short, it looks nothing like it does for someone with an MND, but it is the same spinal condition. It tests the same way, and they take the same drugs.
So both me and my wheelchair basketball buddy both have spasticity. It looks completely different in each of us, because I have a disease that is killing my UMNs, whereas she got shot in the back 25 years ago. Looking at us, a layman would understandably not guess that the same name problem, the same underlying wiring damage, causes her legs to jitter in her wheelchair and mine to turn into unbendable stilts. Nor would we have ever known by swapping war stories. We each know we have spasticity because we went to a doctor and he explained to us that we were spastic, and how that could impact our day to day lives. It is a particular kind of damage to the spine that produces a speed based resistance to passive movement to everyone who has it, as well as a host of very varied inconveniences that aren't standard from person to person.
Does that clear it up?