More accurately, you want the negative/vacuum/low pressure pulse from the divergent cone to arrive around BDC where it will pull fresh charge through the cylinder from the crankcase all the way into the exhaust pipe. About a cylinder's volume into the exhaust pipe. Then, just prior to the exhaust port closing, you want a positive pulse from the convergent cone to arrive, drive the cylinder's worth of charge back into the cylinder and prevent any spill from the cylinder as the port fully closes.
If you are really on it, that positive pulse will reflect off of the closing port, traverse the pipe's length and back, and arrive in the exhaust port just before the descending piston opens it. Thus it will reflect again and head off down the pipe - followed very closely by the fresh positive pulse released on opening of the port. The two pulses in effect combine to make a stronger, broader pulse which augments the whole process.
Back pressure - as it exists in the exhaust system - is determined by the stinger's (the short narrow pipe at the end of an expansion chamber) dimensions and creates back pressure in which the aforementioned pulses can travel (sound doesn't travel in a vacuum) effectively. Overdo it though and the build up of temperature at the port will overheat the piston and create mayhem.
The above describes a very highly tuned engine with a single expansion chamber per cylinder. Multi cylinder (in groups of 3 or 4 cylinders) can use 'cross charging' where the required positive 'back pressure' pulse is supplied from the next cylinder in the group to fire opening its exhaust port and its pulse finding its way to the exhaust port where it is needed to 'seal' the closing port. More to prevent charge spillage that cramming charge in the exhaust pipe back into the cylinder as if there is a negative pulse at BDC, it will not be of the strength of that in an expansion chamber to have drawn charge into the pipe. These are 'compact branch manifold' engines - find them in outboards - as the length of the individual branches determine the timing of pulse delivery to an exhaust port. Short for groups of 3 (and more consistent over the entire rev range - not peaky), longer for groups of 4 cylinders as befits the different (120deg vs 90deg) firing intervals of the two types.
