Wu (2012a) compares běnlái and yuánlái and argues that the essential difference between běnlái and yuánlái lies in the ways how contrast is satisfied: běnái allows for one-way contrast, as pointed out by Wu & Kuo (2012), but yuánlái two-way contrast.
for běnlái, a proposition is true at a time in the past of a reference time, but not true at the reference time, whereas, for yuánlái, a proposition can be true at a time in the past of a reference time, but not true at the reference time, or vice versa.