Just start cutting down, and the moment you think it might be starting to close up start reaming. Once it settles and touches at the top you're good and can simply finish the cut. Once you get the hang of it you'll never go back, hands down the quickest and easiest way to buck stuff. Often you don't even need to roll it to finish, a huge advantage, and you'll eliminate most cutting upwards which is a pain to do with a bigger saw. You don't have to be completely buried either, lead with the tip a bit as you're coming down and then you'll only need to ream the close side, which you can do with your bar pulled out more so you're less likely to get pinched because less of the bar is in the kerf. You can often get away by simply nibbling the sides too to let it settle, and then plunge cut your kerf back open once it's done moving, I'll do that more on smaller stuff since it's easier to get stuck doing smaller stuff because it'll move more. If it ever starts feeling tight ream sideways a bit rather than down, I'm not above even plunge cutting right next to the kerf to give myself more room since I'm not a logger who cares about the wood I'm cutting up.
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