TDST is a central and very important part of the DeMark work. It is the lowest point in a buysetup or the highest in a sellsetup ?link to setup in the countdown page. It is then considered the major support or resistance in the following move.
As long as the move is contained within TDST, that is without a qualified break, the market is considered to be in a range.
Break of the TDST is a momentum thrust and such, the move should continue to complete a full countdown.
With a non-qualified break the move is more likely to reverse but more or less all crashes or meltups are proceeded by a real TDST break.
This differs from traditional TA where the lowest point is always considered to be the last outpost. In DeMark it is instead the TDST, the lowest point in the setup that is the most crucial.
The qualification condition also makes it easier to identify a bigger break. If the market opens up the day after a break, or vice versa, probabilities are big that a sharp reversal is seen.