The phrase "flat palm" means holding your hand open with the fingers stretched out and touching each other, instead of curling or bending them.
Sentences with «flat palm»
But you would stand from behind someone, and you have just nice flat palms, and you just go through the surface, and you touched, you pushed, and you want to feel it come out in the other side. (justinhealth.com)
This big,» he said, measuring the height with the universal flat palm at his waist. (theexpeditioner.com)
Squeeze the two flat ends together with flat palms. (ptinto.com)