This technique allows the user to control the weather in the immediate area, or more specifically, control the water in the surrounding atmosphere.
When Gaiaku combines this technique with the Preta Path's ability to drain chakra, he can negate the effects of weather conditions, such as chakra-sapping rain, and prevent them from being used whilst this technique is in effect, this technique still retains a considerable portion of its strength even when the user doesn't possess the Rinnegan.
Avalon can use this technique utilizing the natural energy in the atmosphere to aid in his manipulation of the atmospheric temperature without expending a significant deal of chakra, as one normally would without it raining or there being a fair amount of water in the air.
By forming (and maintaining) the appropriate hand seal, they can force clouds to amass and force them to produce snow formed with their own chakra, whilst simultaneously drastically reducing the temperature in the encompassing vicinity. So long as they form and hold the hand seal specific to this technique, the user can manipulate the temperature (as an Ice Release user, they'd unable to make the temperature rise any greater than it was when originally manipulated), enabling them to force it to drop far below freezing point, to the point that those unacquainted with consistently cold temperatures (predominantly non-Ice Release users), already chilled to the absolute edge, will develop hoarfrost on their bodies, forcibly develop frostbite, and completely disable most of their bodily processes. The snow will keep on falling until the user uses the seal to cancel it.
To a lesser and more harmless degree the user can simply have it snow without adding adverse effects to it.