When that person comes close to you, your heart starts fluttering, beating faster and harder, and it feels like it might explode out of your chest. Your hands get clammy, and your head feels like it's in a fog, where the only thing you can see, hear, feel, is them. When they touch you, your skin flushes from your head to your toes, and the spot they touched becomes super-sesitive, as if your body desperately wants to feel their touch as much as possible.
Sprung is when you have no control over how you feel for someone; you feel almost obsessed with them. Yet, somehow, it's still not quite love. Eventually, it will turn into love, though, given enough time.