WEEKDAY formula confusion

----M-----------------------------N------------------------------O------------------------------------P
1--Cal Start Date-----------Cal End Date--------------"1 = Sun,2 = Mon…"--------No of specific days
2--27/07/2010----------------15/11/2010----------------2-------------------------------------16


I had this spreadsheet sent to me. In the specific days column is the array formula

{=SUM(--(WEEKDAY(M2-1+ROW(INDIRECT("1:"&TRUNC(N2-M2)+1)))=O2))}

Can someone break this down and explain it to me, because I am LOST!

Many thanks,

Tom