When the right palm is placed over the back of left palm, both palm being in Ardhachandra hasta.
Lets take a look at the viniyoga shloka
etasya viniyogastu sammatau matsyadarshane
It says that this gesture is used to denote a Fish. The Matsya Avatara of Lord Vishnu is also denoted by this gesture. Check out Matysa hasta from 0.20 to 0.39 sec.