D. Marion, P.C. Driscoll, L.E. Kay, P.T. Wingfield, A. Bax, A.M. Gronenborn and G.M. Clore (1989) Biochemistry 28 6150-6156. (Link to Article)
Minimum labelleing: 15N
To start with, an isotropic mixing step transfers magnetisation between 1H spins. Then the magnetisation is transferred to neighbouring 15N nuclei and back to 1H for detection.
This can help identify amino acid types. If assignment is being done using 15N protein, only, it is a must. It can also help with the assignment of side-chain hydrogen atoms, though normally an HCCH-TOCSY or HCCH-COSY spectrum is necessary for unambiguous side-chain hydrogen assignment.