This technique is used by Shima and Fukasaku to fuse with Jiraiya. It maximises the sage chakra for the person they are fused to and allows them to access Sage Mode. This also allows Fukasaku and Shima to continuously gather natural energy for Jiraiya, as he cannot do so himself while on the move, thus eliminating the weakness of exhausting all of his natural energy in battle. Fukasaku and Shima were not able to fuse with Naruto Uzumaki because Kurama rejected them.
The name of this technique is a pun. Ryōsei (両生) is the Japanese word for "amphibian", but the kanji literally mean "both lives" or "two lives", referring to how this technique fuses two living beings.