As described on the page I linked to, each timer has an A and B side, and those sides can be commanded to toggle/set/clear a fixed pin, as per this table:
Timer 0
input T0 pin 6 (D4)
output OC0A pin 12 (D6)
output OC0B pin 11 (D5)
Timer 1
input T1 pin 11 (D5)
output OC1A pin 15 (D9)
output OC1B pin 16 (D10)
Timer 2
output OC2A pin 17 (D11)
output OC2B pin 5 (D3)
So, OC1A can affect Arduino pin 9 (pin 15 on the actual chip).
I mean, the goal here is to enable the timer/counter interrupt mask register (TIMSK1) so as to enable timer interrupts and this is the point of that line.
Yes, but you can't process interrupts that fast, as I said. The hardware however can toggle the pin at any reasonable rate. Instead of using interrupts, let the hardware do it. In any case, interrupts can be out a bit if another interrupt happens to be active at the time (eg. Timer 0).