Puzzle of the week #148

Chess Diagram: 


The more we practice combinations, the better we get at it. Remember, combinations do not happen out of the blue; a combination becomes available when the position is ready for it. This is the reason why we always have to analyse any position we have in front of us; this way we can understand if it is the right time for a combination or not. It is the right time for a combination in this position. Your tasks:
a) Analyse the position and explain why is this the right time for a combination;
b) Black to move and win. Find the winning combination.

Total available points for this puzzle is 20. The answers will be published next week together with puzzle #149.

Puzzle #147 solution:
Based on Oll - Orest, 1985, Facebook chess puzzle. I have received quite a few answersand this is a very good sign. It was not easy to choose the best answer, but in the end have chosen James. Here it is:
"1.Rh7!! ... there are many possibilities,
1... Re7?? 2.Rh8#

or 1... Qd7 2.Rxd7 Rxd7 3.Rxf6+ Ke7 (forced) 4.Qf7#

or 1... Rd7 2.Rxf6+ Qxf6 (forced) 3.Qxf6+ Rf7 4.Qxf7#

or 1.... Rd7 2.Rxf6+ Qxf6 (forced) 3.Qxf6+ Kg8 4.Qh8# or 4.Rh8#"
Thank you James! Very well done.

Correct solutions:
Frank, Ziyao, James, Alex - 10 points
Philip - 9 points
Harmony - 8 points
Jeffrey, Danny, Leo - 6 points
Leroy - 5 points
Kevin - 1 point


Ziyao - 27 points
Frank - 23 points
Harmony - 21 points
James - 20 points
Alex, Jeffrey, Danny - 19 points
Leo - 16 points
Danny, Karl - 13 points
Kevin - 11 point
Leroy - 10 points
Philip - 9 points


The final result counts