What metaphors are used in Bedtime Story by George MacBeth?

Round eyes roll round and forked arms waver Huge as the rough trunks.

These lines are ambiguous at best and can be seen as metaphors for saws and other heavy equipment, but what would "forked arms ... Huge as the rough trunks" be doing above him? At this point, the description breaks down and we are left to make sense of imagery which does not seem to fit.