A proper grip will reduce muzzle jump or flip. The muzzle of your Shield is probably jumping up and your thumb is staying where it was, therefore appearing to slide down. With a proper grip, your thumb should stay in the same location on the fame of your pistol.
There is a point or axis of rotation (upward) when a pistol recoils. The farther back your grip, the more rearward that axis is, and the more muzzle jump you will get. The more forward your grip, the more forward that axis of rotation. The more toward the center of the gun from back to front that axis is, the more you can control muzzle rise with a good grip.
Having a high grip has a similar influence on the axis, moving it up and therefore producing less rotation.
Don't bother to do this, but imagine gripping the gun with one hand, purposefully low, with your hand as much behind the gun as possible, and with a fairly loose grip. What do you think will happen to the muzzle if you shot?
Using your shooting hand alone, mostly behind the grip, will produce a rear located axis. You will get muzzle flip. This cannot be avoided.
Adding the second hand to the gun changes things dramatically if done correctly--moving the axis forward and higher. If the second hand is low, toward the rear, and too relaxed, it will have less effect on muzzle flip. While the second hand thumb is not really the critical component of a good grip, it is an indicator that the rest of the grip is working correctly.
The two keys to the second hand reducing muzzle flip are:
1. The meat of the second hand thumb fills the void on the grip between the shooting hand finger tips and the shooting hand thumb meat. This gives close to 100% grip coverage and control. There should be good pressure from both hands all around the grip. This may be difficult to do on small guns as the hands tend to be too large to properly grip. Practice a lot just grabbing and holding the gun without shooting until it feels "right", as secure as possible.
2. The fingers of the support hand do more to control muzzle flip than any other part of the grip. Their downward and rearward pressure on the grip (through the shooting hand fingers) make the biggest difference. Like riding a bike, you will find a grip that feels just right, with comfortable control, if you keep practicing without even firing.
The placement of your thumb(s) pointed at the target actually helps you to point the gun more naturally, but they don't contribute to recoil management. The more forward that second hand thumb is, the easier it is to point toward the target.
What I find helps with that is to rotate the second hand thumb meat upward as high on the grip as possible. This allows me to comfortably lay my second hand thumb along the frame right at the juncture of the slide.
As well, I have found it helps to rotate my shooting hand grip slightly around the rear of the grip so it's thumb can rest on top of the rear of the second hand thumb and along the frame (thus pointing at the target). On a small gun like the Shield this also improves trigger finger placement so you use the end pad rather than the joint of your finger on the trigger. Part of your shooting hand will come off the grip slightly just behind the trigger. That's OK. Your fingers pulling the gun back into the rear part of the grip--your palm and thumb--is what will give you the best stability with just one hand.
After trying to put all these elements together for a few days practicing at home, one day everything just felt right. I went and shot. The gun pointed naturally and quickly, and muzzle rise, even with the Shield, was greatly reduced. My hands and thumbs stayed in place. When I got this same grip on my FS M&P 9, the gun hardly moved upon firing.
Sorry I don't have a photo, but this grip is the same as what you will see in instructional videos of the high, forward grip. Done correctly, it works to keep the gun steady and pointed in at the target, including immediately after recoil. Your thumbs will stay in place along the side of the gun.
I hope this doesn't sound too complex. The videos I have seen show the grip but do not explain the elements very well. Good shooting!