GR translation: seized out of the last ruffling of a great sigh

[Mike Jing]

V342.17-24, P347.18-26   But Snake here is not so much Midnight-wild
as methodically homicidal. Worse, he’s unpredictable. When you go to
ride him he may be indifferent, or docile as a maiden. But then again,
with no warning, seized out of the last ruffling of a great sigh, he
could manage to kill you simply as the gesture of a hoof, the serpent
tuck of a head toward the exact moment and spot on the ground that
you’ll cease to live. No way to tell: for months he can be no trouble
at all. So far he’s ignored Tchitcherine. But he’s tried for Džaqyp
Qulan three times.

What does "seized" mean here? And is the sigh the horse's?
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Pynchon-l / http://www.waste.org/mail/?list=pynchon-l