I have not seen other owners use these very often, but I find they are very useful in hunting and for a dog in the home.
A soft, single whistle note through my teeth with a hand extended toward the dog showing my palm like a policeman stopping traffic means SIT
Several soft whistle notes through my teeth with the last note longer with an open hand by my left knee means COME.
Both of the above are similar to the loud whistle commands I use for sit and come.
I have used these in upland and waterfowl hunting situations and in the home to control a dog. The hand signals were especially useful when our older dogs lost their hearing. Our current dog learned them just being in the home with the other dog. She needed just a little reinforcement in hunting situations after she was 8 months old.