I once took a course, 30 hours, we jumped on a professional rectangular trampoline without vertical safety nets, supervised. I too was worried about spinal injuries. According to my teachers, most injuries do happen by jumping out of the bounds or impromper landing technique (taking the landing shock the spine by not angling hip/body correctly). With that said: Even without flips it was major Fun. Very fast learning curve (and I am a very slow learner with regards to complex coordinations), the freefalling... Also, flips are an advanced technique, we only attempted them by the end of the course; I felt quite safe by then from breaking my neck, I had learned to control the fall and the spinning. Please note that this risk assessment is based on me weighing 65kgs.