The SFN should fit tight on it’s own and not move. It’s function is not to ‘apply tension’ but to be solid enough for the top bolt to take the play out of headset bearings and no more.
Yes, I always thought that it was to preload the bearings??
yep then when you have applied the preload (which is via the stem to the headset), you lock the stem in position using the stem clamp bolts and this then retains/fixed the preload position you achieved.
once you have bolted up the stem, you could remove the top cap and bolt and it would have no effect on the headset adjustment….
Unless your stem is not done up properly and it slips…