I have worked in Scrum team and i assure it is no way Waterfall model, rather mix of and more near to iterative model, v-model, spiral model,.. The rough estimation is usually done for a story first by product owner. Say a story is estimated with 8 hours of task. Then we distribute cards to team which have 7 cards each with 1,2,3,5,8,13,20. Each member tells his estimate for work. One on extreme side usually justify their estimates and we come to conclusion by discussion.