winglets

OK, I can get the theory (OR can I?) with winglets to reduce wingtip vortices.

Surely the best direction for the tip is DOWN!, I understand this may be difficult for actually making, especially with gliders and groundloops, aesthetics, etc, but can someone explain why they are pointing up?

Consider this..
The HIGH pressure area is UNDER the wing, we want to keep this from migrating to the LOW pressure area ABOVE the wing. Our current wingtip style will restrict the LOW pressure from bleeding sideways (and why would it anyway) yet the majority (?) of our lift is generated from the HIGH pressure BELOW the wing.

This high pressure is what we want to retain and utilise, yet it is still allowed to bleed out sideways. Sure the current shape of wingtip will reduce the vortices caused by the two pressures meeting, and thus the drag, but it wont increase the lift available by retaining the HIGH pressure UNDER the wing, where it is most useful.

I also note the return of the "plate" style winglet on latest jetliners, half above, half below the wingtip, instead of a large winglet.

Enlighten me, O knowledgable ones!

Wayne