kneading and shaking

Hello. My male cat who is 3-4 years old started living with me about 2 months ago. He previously lived w/ my boyfriend. Since being here, he has begun a new behavior of kneading and pushing on my lap and chest. He never did this to anyone else before including me while at my boyfriends house. I understand that this is some sort of compliment when they do this. But the part I don't get is that his body shakes, he arches his back, and contracts his buttox area while doing it. I'm worried that he'll urinate or worse on me. If I try to push him off, he meows in this high pitched voice that he doesn't use at any other time. Last night I let him do it as a test for about 15 minutes on a towel on my lap until he exhausted himself and jumped off to go lay down. I don't know what that is supposed to signify.....do you?
Thanks.

When you say his body shakes, do you mean that it kinda vibrates? Because my Max does that... I believe (and someone will come by to correct me, I'm sure) that kneading means that they are either insecure or content. I know Max always kneads and "vibrates" and arches his back when I'm giving him a good belly rub... I know he enjoys it so I take it as a good thing.

Shaking as in, he physically trembles. His strength seems very unstable and weak. His footing isn't very good and his paws slip a lot. (probably because he's trying not to hurt me with his nails) He does purr loud too!