This is a murder-suicidal technique. The user creates threads of chakra that are used to connect the user's heart to another person's. As long as the two are connected, any internal damage done to one will also be done to the other. This ultimately means that, if one person dies, so will the other. Only a kunai wrapped in a special Fūma clan tag can safely cut the threads and free the two.
It is mentioned that the threads are so resilient that even Tsunade, with her fearsome strength, cannot break them.