legs of

leg 有“阶段”的意思.英文释义是 a particular stage of a journey,competition or activity.
例句 The last leg of the race was Paris to London.最后一段赛程是从巴黎到伦敦.
spread可以做名词,“传播”.
这里的legs是复数,所以我想,应该是“传播的各个阶段”