In general, the "Palms Down" gesture is to be avoided as it is universally negatively received by audiences. It is interpreted as aggressive and excessively dominant. A variation on this is "Palms Down - Open Fingers". However when the fingers are open while the palm is down, the gesture has significantly less negative effect. Also, one palm down is much less negative than two. Still, remember to use the "Palms Up" gesture when possible.