What is the special way of saying ...?

The bloke who talks about such things on the radio in my part of the world calls it contrarianism.

I see. But I would like to have an expression that expresses the idea of buying stocks or properties or assests at the nadir of economy. I hope you understand me.

You're probably looking for a technical economics term, and I can't help you.

His contrarianism is referring to the general stance that you buy stocks when prices are low and everybody is panicking and sell when prices are high and everybody is complacent, contrary to what intuition might suggest.

speculation




From investopedia:

Speculation
What Does Speculation Mean?
The process of selecting investments with higher risk in order to profit from an anticipated price movement. Investopedia explains Speculation
Speculation should not be considered purely a form of gambling, as speculators do make an informed decision before choosing to acquire the additional risks. Additionally, speculation cannot be categorized as a traditional investment because the acquired risk is higher than average.

More sophisticated investors will also use a hedging strategy in combination with their speculative investment in order to limit potential losses.

"buy at the bottom"