What kind of motion did the younger monk perform in The Things They Carried?

The younger monk makes a washing motion with his hands.

The men in the Company first notice the peculiar motion the younger monk makes when they are digging their foxholes in the yard of the "almost abandoned" pagoda and shrine.  The monks speak "almost no English", and although the soldiers do not feel right about establishing their camp on sacred grounds, the monks do not seem particularly displeased.  Each morning the monks bring the Company buckets of fresh water with which to bathe.  On one occasion, they present the men with ripe watermelons from their garden, and when the soldiers have finished enjoying them, the younger monk again makes the strange washing motion with his hands. 

The men never do figure out what the washing motion means, but since it appears to be a polite gesture, one of them, Henry Dobbins, performs the motion in return.  The men discuss their feelings on religion and the church and being kind to one another, and conclude that, for a reason that is not clear to them but which they all feel, it is wrong to set up their camp at a church, just as it is right to establish good relations and nurture what they have in common with the gentle monks, to "be nice...treat them decent, you know?" ("Church").