He desired to sacrifice again with a greater sacrifice. He toiled and performed penance. And while he toiled and performed penance, glorious power went out of him. Verily glorious power means the senses (prana). Then when the senses had gone out, the body took to swelling (sva-yitum), and mind was in the body.

In plain words: Death wanted to perform an even bigger sacrifice, so he worked hard and practiced strict discipline. As he did, his strength, meaning his senses, left him, his body began to swell, and his mind stayed inside it.