In the Hull-White model, the drift of the short rate is θ(t)−art\theta(t) - a r_t, and aa is the speed of mean reversion. When the rate is above the level θ(t)/a\theta(t)/a it is pulled down, and when it is below, it is pulled up.

Consider a hypothetical example in which the level is 2% and a=0.1a = 0.1. A short rate of 3% has a drift of −0.1 percentage points per year (0.1 × (2% − 3%)), and a short rate of 1% has a drift of +0.1 percentage points.

The larger aa is, the stronger the pull, and the smaller the spread of the short rate in the distant future. Mean reversion does not make the rate settle at the level. The random movements keep it moving above and below the level.