What does a trough in a tsunami do?

Answer 1

When the first part of a tsunami to reach land is a trough rather than a crest of the wave, the water along the shoreline may recede dramatically, exposing areas that are normally always submerged. This can serve as an advance warning of the approach crest of the tsunami, although the warning arrives only a very short time before the crest, which typically arrives seconds to minutes later.[2] Although in the 2004 tsunami in the Indian Ocean the sea receding was not reported on the African coast or any other western coasts that it hit, when the tsunami approached from the east.

Answer 2

Troughs precede and follow the actual tsunami wave(s) - they are the low water points in front of and behind the crest.

All waves have them - just imagine them being 100 - 1000 times larger.

The preceding trough is what causes the water to recede from the coast.