Underflip.: As usual, pop then set the trick.  If you spin left, start a slow spin (theoretically, you're shooting for 180 on an under 5).  Look towards the outside edge of your left boot almost like you're grabbing safety.  This will set the lincoln part of the flip.  Continue the lincoln and try your best not to panic.  You will be very slowly spinning, and flipping a lincoln fairly quickly.  Around 360/450 you'll start to see the landing... however if you look closely you should see the knuckle most of the way but you may miss it the first few times.  Stomp that shit at 5 like a playa.  Underflip 5's are very very easy and smooth if you set them correctly.  An underflip is purely derived from takeoff.