At the start, the use of unconventional tools very much came down to learning on the job. Most of the instruments were new when they were introduced and beforehand their effects were to a large extent unknown.
The uncertainty was also due to the difficult circumstances in which central banks had to operate: financial intermediation was impaired and inflation dropped significantly below our aim of below but close to 2%. To address this, central banks started communicating more clearly on future policy rates, took policy rates into negative territory and engaged in balance sheet policies to complement conventional policy rate cuts.