Tomas DeMark在技术分析新义td回撤一章提到td回撤弧
根据http://wealthv.com/learn/chartfilter/td_arcs.htm的描述，弧与预测的阻力线与支撑线之间存在先后关系
1）股价先达到阻力线或支撑线，后到弧，表明此时市场动能或趋势强劲，将更可能突破到更高水平的阻力线/支撑线
2）同时到达，表明市场动能耗竭，不大可能突破后期阻力线或支撑线。
原文描述：
If the price retraces 38 percentage points of the previous move in less than 38% of the period that generated the last move, the price has a greater chance of retracing further to the 62% level.
The key to TD Arcs is the speed with which the first retracement level is reached and broken (or not) by the price. If the 38% level is reached and broken before the 38% Arc, there‘s a strong tendency for the momentum to carry the price up through the next retracement level at 62%. As the example above demonstrates, however, if this doesn‘t occur quickly enough (before the price reaches the 38% Arc) the momentum tends to lag, the retracement becomes exhausted, and the market once again follows the overall long-term trend.