Perhaps another way of explaining it. In box 2, all the cells with candidate 1's are in column 5, so all other candidate 1's in c5 can be excluded (r5c5, r7c5 & r9c5). That leaves all the cells with candidate 1's for row 9 in box 7 (r9c2&3). Since box 7 & row 9 must contain a 1, all other candidate 1's in box 7 can be excluded, leaving r8c1 with a single possibility.

Starting from the position initially posted, I used Mandatory
Pairs to record additional information before proceeding to
derivation of the candidates. Normally I would have the M/P
data marked already but some had been eliminated on my
own solution which I completed on 10th.

a) Box 8 must have '1' in row 7 or row 8
(as r9c5 is excluded by pair in box 2)
b) Box8 AND Box 9 now have '1' restricted
to row 7 or row 8. By one of the M/P
technique rules the '1' in box 7 MUST be
in row 9. M/P in r9c2 and r9c3.
c) Column 1 has seven cells resolved. The
two missing values are '1' and '2'.
From 'b' there cannot be a '1' in col 1
on row 8 and so the '1' must be in row 2.
d) This leaves only one cell in column 1 and
so r8c1 MUST have value '2'.

When I solved it originally, I resorted to use of candidate profiles
but on this second look, it has been solved WITHOUT having to
use candidates at all - BUT this was a different puzzle in that
some cells not given in the original puzzle had been resolved
before the query was raised under this topic.

One of the paradoxes of SamGJ's grading system is that "V.Hard" is
often more amenable to Mandatory Pairs than are the plain "Hard"!

First time responding so not fully in tune with the terminology and unsure how the words will translate (I am planning to create in Word, then paste into response field and then look at what shows up).

I had difficulty with this puzzle also. The way the solution came to me was when ‘4’ fell into r2c6 then:

Ø The ‘1’s’ in box 2 at r1c5 and r2c5 were paired (hence no other ‘1’s’ in c5)
Ø Hence, ‘1’ in r9c5 was eliminated creating paired set of ‘1’s’ in row 9 in cells r9c2 and r9c3
Ø Hence, eliminating the ‘1’ in the ‘12’ pair in box 7 at r8c1. This ‘2’ in r8c1..
Ø Eliminated all ‘2’s’ in row 8 leaving the only remaining ‘2’ in box 9 sitting at r9c9