Although the chin-na application is generally regarded as the primary application, per placing the palm/fingers of the off-hand on the wrist joint of the main hand, when I first asked my teacher, this was the application I was given.
Needle to Sea bottom is a very effective finger strike into the cavity between the hip and groin.
It also works as a push to displace the hip joint to disrupt the opponent's root and control their body.
If one does not have sufficient fa jin for the finger strike, the push is the alternative.
This target is part of the reason the movement is generally practiced by bending the waist such that the main hand goes almost to the ground.
(I started practicing it in the manner of the application, countering and pushing the fingers forward at hip level, as my teacher demonstrated, but not when practicing as part of a group on in group exhibition, b/c my teacher didn't do it that way in those contexts.)
- The strike involves pushing into the joint and then downward, focusing only at the end of the technique, into the downward diagonal vector.
You also see that technique in some pakua strikes to the hip. (Supposedly these pakua strikes can be used to dislocate the hip with sufficient focusing, but I haven't validated this personally.)
- Can also be applied to the front of the shoulder joint.
In this case, trapping the opponent's hand on the wrist, and lifting up to disrupt their balance, sets them up for the finger strike or push into the shoulder joint.