Loosely speaking, a transient denotes a part of a signal with sudden changes. Think of a constant level signal followed by a damped sine, for instance.
Analyzing a transient with windows such as the Hann or Hamming may affect the transient energy a lot, as these windows decay too fast to small values. Thus, it is sometimes said that it is better to use a rectangular window, to have a constant amplitude change across the time interval.
However, I plead guilty of using windows with very short raised power cosine tapers, such as the one below, to allow the windowed signal to fall down to 0 at the edges, to ease FFT calculations for instance.
What is happening on a real-world ultrasound signal? On top, a part of a transient ultrasound. On the center, a rectangular window, an Hamming one and the custom one I talked about above. On the bottom, the windowed signals. The one with the custom window looks closer to the original than the Hamming-windowed one.