how to : find a simple way to "fix", or anchor, only hands on a trigger

There are several problems you may hit in this process. But a few suggestions to get you going a bit further:

• If you want the head movements to not move the body, consider using a independent layer for the head. Independence stops a child from impacting the parent. Otherwise you need to put pins or similar on the shoulders and base of neck to stop movement.
• If you want to move hands in front of the table and the body behind the table, the easiest way is to effectively make the table the characters "pants", then use sticks and pins to lock it down. E.g. put a stick across the top edge of the table, then add some pins to stop it moving.
• If the head cannot be independent, then you need to use sticks and pins again to stop the shoulders from moving.
• If you want the head and shoulders to move, but the hands to stay on the table, that is hard actually. You might be able to put a dragger on the hands, then record a take of the hands after the body movement has been recorded. This should offset the shoulders movement and keep the hands safe for the duration of the recording.

But it is quite possible what you want cannot be done. CH does not allow a layer to have to "parents" controlling it. So you cannot do things like an arm where one end is attached to the shoulder, and the other end is attached to something like a desk. Just not available.