There are 2 things you need to understand.
1st, NEVER let the saw touch the dirt. It will dull it instantally.
2nd, you have to know what part of the log is in tension and what part is in compression.
If the bottom of the log is in tension and top in compression, and you cut top down, you will pinch, as you have figured out.
IF there is clearence underneath, cut from the top JUST unitl you start to see the gap close up. STOP. Pull out and undercut.
The reverse is true it the top is in tension and bottom is in compression.
Just be MINDFUL at ALL times when cutting. Know WHERE things are going to move/settle once you make the cut.
And honestly, it is very difficult cutting large trees without some equipment, like a tractor/loader. Because what we do, in a case like yours where you cant cut all the way through because of dirt/pinching: cut as much as you can until the gap closes or you have to stop to stay out of dirt, then move on to the next cut, and so on. ONCE you have all the cuts made, use the tractor to "roll" the log over.
And if you dont have equipment, you can use a peavy or cant hook. you can google both to see what I am talking about. But they dont work well on really LARGE logs.
And a final tip, always start with the smallest part of the tree. NEVER start cutting at the base first. Cut up all the limbs and get the brush out of the way. When you get ALL the limbs cut that you possibly can, then start on the base. Reasons for this are:
1. Limbs in the air are easy to cut as opposed to on the ground.
2. It makes the base/trunk less likely to move when cutting it, due to limbs sticking out and up in the air.
3. It makes the base easier to roll IF you cant cut all the way through, as described above.
I know all that is alot to take in. IF you are unsure about anything, or have more specific questions, please just ask. A chainsaw is a VERY dangerous tool in the hands of an in-experienced operator. And that is NOT meant to insult you. It is meant to encourage you to PLEASE ask if you have ANY doubts at all about what you are doing. I would rather you ask a "stupid" question, rather than just try something and end up getting hurt.:thumbsup: