This posture is described in the 19th century text Sritattvanidhi as follows:
“Place the hands and the forearms up to the elbow on the ground. Draw the knees up to the navel and remain in that position. This is catakasana, the sparrow“.
A depiction of this posture does not seem readily available in the modern day (i.e., it does not seem to be practiced widely in the modern day). Today’s version of sparrow pose seems to be more like this.