A valve with a sliding part that controls the extent of the aperture.The gate valve attaches to the other end of the pipe....
Plumbed into the down hose with a gate valve in-line, it's adjusted so it can only lower the load slowly.
After the seacock or gate valve is closed, remove the hose temporarily so that it drains and then use an absorbent cloth or turkey baster to eliminate any residual water in the nipple.