This effect is frequently discussed in the context of solar time series. In particular, it a relationship between soft x-ray (thermal emission) and hard x-ray (nonthermal emission) from the Sun.
The Neupert Effect describes the commonly observed phenomenon that hard X-rays occur predominantly during the rise phase of soft X-rays during a solar flare, as illustrated by the graph below:
In the graph, the first peak is that of the hard X-rays, which you can see occurs during the rise of the other line which represents the soft X-rays.
Following the previous answer stating that the HXR emission occurs during rise phase of the flare as evidenced by the full-Sun SXR flux observed i.e. by GOES in 1-8 Angstrom, I would add that the HXR maximum is typically observed during the strongest increase of the SXR flux, i.e., during d(flux_SXR)/dt = max.
What's more, the Neupert effect is often stated that the hard X-rays look like the time derivative of the soft X-rays. The physical significance of that is it suggests that the presence of hard X-rays relates directly to the energization of soft X-ray gas. It supports the common model that hard X-rays are a byproduct of particle acceleration, and the accelerated particles deposit heat that gradually leaks out as soft X-rays.