Alusec remembered what happened when he was jumping out of the sea of lightning storms. At that time he had run out of source energy and he wouldn't have been able to get to the location where the soul realm was constructed when he leaped out with his last strand of source energy.
If not for Erin's sword that was waiting for him at the exact location where his leap would reach the limit, he would have failed to get there.
Alusec had already put two and two together and now understood how that happened. Looking at it clearly, it was impossible for them to know the exact place when his jump would lose power and would start falling.
It was because of Rin. After using that technique, Rin was able to peek into the future to know exactly what was going to happen.
She saw how it would have ended to Alusec's disaster. She borrowed power from her future self using that technique which meant her future self had the ability to do just that.
