Denial

Im the Thorn EMI game 'Tank Commander', the instructions say

press :-
...
f7 for interlace

Doing so doesn't seem to change anything as far as I can tell.
Does anybody know what the purpose of this is or was meant to be?
(I am fully aware of what interlace is, and do not need a description of interlace itself. I want to know what the key press does).

F7 toggles bit 7 of VIC register $9000. Nothing more to that. As you're on a PAL machine, and interlace is not implemented there, you don't see any effect.

More background, here: Commodore cartridges anti-screen flutter key?

Ah, that explains it, thanks!