Looking at the tree in the first 2 pics, cut a slit in the bottom first (if you can get your saw in there, then cut from the top down in a / direction, matching up with the bottom cut. The root ball is going to want to sit back back, which mean its going to want to go up. By cutting it at that "/" angle, the root ball won't pinch the saw when it tries to lift.
Be careful, if that thing starts to lift early it'll likely grab your saw and lift right at the cut before the wood is cut, leaving a hinge and picking up the last leader. I usually cut the root ball off first, sometimes there is a lot of tension trying to pull them back down, and it'll lift the tree up once you get the majority of it cut up. Happened to me once, caught my belt on a branch, and lifted me 10 feet in the air. Boy did I look like an idiot calling for the woman to bring a ladder from my cell phone. I'll never cut from the top down again (if the root ball is sitting upright anyways).