I don't see a lot of difference between grabbing the wrist (gyaku-hanmi katate-dori) and chudan tsuki (punch to stomach). In TNH I always grab like I'm going to go through tori's centre, just as I would in a tsuki attack
If tori doesn't blend with and re-direct the energy I'm giving him, he's going to get hit in the stomach. (Gently with beginners. Hard with yudansha
)
Try attacking like that - you can easily align your hand with both your own centre and with tori's to make an effective attack - think of it as your centre attacking tori's centre.
Hope this helps,
Have fun!
Ruth