After entering the spatial rift, Rien's lovers immediately started attacking the demons who were there while Rein took out the modified rain stone and used that. He was not too worried about this as even if the rain stone failed he would still be able to accomplish what he came here to do. So being able to use the Rain Gain was just like an extra for Rein at this point of time.
As soon as the rain stone was crushed, water started to fall from the sky but it lasted for just a moment and immediately disappeared. So this rain stone activated normally but the rift immediately got rid of the rain. But still Rein was able to gain the boost from the rain gain in that short period of time which would allow him to defeat anyone in that period.
Even though that period of time was very short, for a Rein boosted by rain gain, that much time was enough to end a battle or two. So Rein was still satisfied with the outcome.
