Everyone with a position that calls for an immediate implementation of a thing that actually requires several steps to occur first. Essentially anyone with a "press the communism button" mentality instead of a "communism is a movement over time to abolish the old order of things" mentality.
For example, someone demanding that China implement a fully socialist command economy like the USSR. This analysis is deeply flawed because the conditions China exists in are not the same as the USSR, China does not have 100% of the resources it requires within its landmass, China requires access to the global market for many things, notably 75% of its oil requirements are imported. If China did not have a market economy it would be cut off from the global markets as it would serve no value to the international bourgeoisie who function as a faction that wants China to have access to those markets so they can profit from investing in it. China would be sanctioned and it would undergo enormous hardship. They must first achieve self sufficiency before this is even possible, getting off oil, getting off uranium (also a key import), among other things. Thorium reactors and solar are concrete steps towards this but these are just a couple of major examples.
It is an ultraleft position to hold the view that China should press the socialist economy button. The ultra left is not considering the conditions properly when they take this position. A development process must occur first to bring about the conditions where that action is possible.
Almost every position that anyone would call ultra will usually fit into this.

