While the hands are in chakra hasta, bend all the fingers (except the thumb and the little fingers) and hold the palms tight.
Lets take a look at the viniyoga shloka
The shloka says that this hasta is used to denote a Tortoise. The Kurma Avatar of Lord Vishnu is also denoted by this Hasta. Check out the Kurma hasta from 0.41 to 0.55 sec of the video below.