CRT monitors internally use an electron gun which just fires electrons at the phosporous screen (from, the back, obviously, and the whole assembly is one big vacuum chamber with the phosporous screen at the front and the electron gun at the back) using magnets to twist the eletcron stream left/right and up/down.

In practice the way it was used was to point it to the start of a line were it would start moving to the other side, then after a few clock ticks start sending the line data and then after as many clock ticks as there were points on the line, stop for a few ticks and then swipe it to the start of the next line (and there was a wait period for this too).

Back in those days, when configuring X you actually configured all this in a text file, low level (literally the clock frequency, total lines, total points per line, empty lines before sending data - top of the screen - and after sending data as well as OFF ticks from start of line before sending data and after sending data) for each resolution you wanted to have.

All this let you defined your own resolutions and even shift the whole image horizontally or vertically to your hearts content (well, there were limitations on things like the min and max supported clock frequency of the monitor and such). All that freedom also meant that you could exceed the capabilities of the monitor and even break it.