Could you please provide a source for not pouring underhand? Just because something is done in a manner of taharah doesn’t necessariy make it ossur.
Shalom, The Chevra Kadisha (burial members) have a custom at their homes not to do actions that are done at the Taharh, before burial. One of these actions is not to pour a drink underhand, since this is how they pour water at the Tahara. This custom has no source, and only members of the Chevra Kadisha do it. Other people do not need to keep this custom. (Answered by Rabbi Ya'akov Ruza)