A general plan by White is 1.Ra5#, but 1…Kxd6. At moment White has not 1.Rd6. Therefore a position of Bb2 needs in improvement:

1. Be5! (>2.Ra5#)

1… Kb5 2. Bc7! Kc5.

Again White has not 3.Rd6? Rh5! So, 5^th line must be interfered

3. Shf5! gf5, and now

4. Rd6! is sound.

4… Rd4. A field d4 is blocked, white Rook comes back:

5. Rc6+ Kb5 6. Ra6! Kc5, and general plan is working –

7. Ra5#!