Really hard to describe, but it's not normal back pain, I've had a bad back at times, sometimes really bad but this was just something else, felt like spinal surgery, no mistaking it not for labour!
Edit, it still came in waves, like contractions, that got closer together so I guess you'd go in when they were close enough together. I had meconium in my waters so went in then, never had to worry about the getting there in time side!