The Side kick series hand placement is practiced in the Classical position. The bottom palm of hand is pressed into the mat and in line with the waist. The shoulder blades are actively drawn down the back helping to support the rib cage.
However, people are spending more time at their computers and wreaking havoc on the wrists with repetitive wrist movements. The Stability position may be a more effective position because the wrist joint is stablized and supported using the fist.
To advance this series with the Challenge position, the bottom hand is slightly wrapped around the waist and lower ribs, to maintain a square ribcage, opposed to the ribs usually sinking into the mat. The shoulder blade connection on both sides is imperative to keep the spine from dipping down.
In my presentation of the full Intermediate mat, the first 3 exercises I incorporate the wrapping of the waist to support the bottom ribs, and the fist is used for the Bicycle.
Try what works for you. Classical, if there are no wrist issues, Stability, if there are, and the Challenge position to strengthen the ribcage. The final and most challenging hand placement are the hands behind the base of the skull with the elbows wide.