He slammed the counter with the flat of his hand.

他用手掌猛击柜台。

《柯林斯英汉双解大词典》

He beat on the door with the flat of his hand.

他用手掌打门。

《牛津词典》

Using the left hand pull the top section down in order to line its edge with that of the under section (needle still in the down position) which remains flat.

为了使上层裁片的边与下层裁片的边成一排，用左手将上层裁片往下拉，这样可保持平整。

youdao

One of the hands keeps a flat hand shape and it just kind of lightly moves back and forth as if it were a flickering image on a screen."

VOA: special.2011.04.13

Over the centuries, the river had slowly carried away mountainsides, tumbling and cracking rock, reducing boulders into a rainbow of smooth, flat stones the size of a child's hand.

CNN: The seven stones of simpler living

But fee-only financial planning includes other methods of charging clients such as a flat retainer, an hourly rate or a charge specific to the task at hand.

FORBES: Fee-Only Financial Planner: What's the Difference?