What is a 'left-handed invitation'?

What exactly is a ‘left-handed invitation’?[YSaerTTEW443543]

TOEFL listening lectures: A university lecture in Biology[YSaerTTEW443543]

Without any further context, I’d say that using ‘left-handed’ puts the sincerity of the invitation in doubt.

Makes sense, Amy. Thanks a lot for responding so fast.[YSaerTTEW443543]

TOEFL listening lectures: Why are red crosses erected on Tanna Island?[YSaerTTEW443543]