On a pedal harp you would have to kneel down or something to rest your left arm on the board, Maria.
With the right arm, you’re not actually pressing the arm on the soundboard, it just touches it. In any method your arm needs to support the hand in order for you to have the fullest range of motion for your fingers and for moving up and down the range of strings. If it doesn’t, the weight of your arm pulls your fingers away from where you want them to be and you’re working against yourself.
If you actually press on the edge of the soundboard as I’ve seen beginners and self-taught harpists do sometimes, you’re asking for trouble eventually from compressing the carpal and/or ulnar tunnels.
If your left arm is touching the soundboard on a pedal harp that’s the right size for you (there may be exceptions with tiny people playing very big harps, for example), you generally aren’t giving the hand the support it needs, and you almost certainly aren’t playing in the optimum area of the strings (unless you’re doing a special effect or something.)