Boltek Lightning/2000 - Manual (Page 234)

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Another type of intracloud stroke seems to crawl (spider
lightning) below the bottoms of the stratiform area or in other
cases below the anvil formations of thunderstorms. This type
of stroke can produce a movement of charge that can be seen
as –IC or as +IC depending on whether the charge is carried
to or away from the detector, and in some cases be
undetected if the orientation of the charge movement relative
to the detector is entirely horizontal. This occurs as an
extended stroke between the charge layers in the stratiform
area. This is shown in figure 8.
The stratiform charge layers can also lead to a CG stroke
(usually a +CG) as a result of the spider lightning stroke. This
is shown in figure 9.