By playing on 100 fps and 75hz the image displayed on the monitor will be much like playing on 75 fps but your gun recoil will be just like at 100 fps.With your fps set at 75 it will look the same but your gun recoil is going to be fucked up do to lower fps.
For a smooth gaming experience on 75hz, try playing on developer 1 and putting fps_max at 120 or 140 :)
Remember that developer 1 removes "[x] Read Only" from your .cfg files. So you have to back up those settings.
its like 100 fps is 100 frames generated per second and 75hz is 75 picture your monitor refresh per second.
fps is number of frame your gpu can calculate and display
where as hz(hertz) is number of picture your monitor capable to refresh per second.
Look at it this way: your gpu sends the screen 100 frames while the screen only refreshes 75 times. This means that some of those 100frames will collide at certain points because there are more frames than there is hertz. -> that's how you get tearing on your lcd.
even though your monitor only display 75 frames per second(75 hertz) your game still calculate 100 fps so it doesn't matter how fast your monitor display the recoil still the same.
the only real difference is that you can see the image being refreshed at it's fastest which means faster reaction time.
it doesn't effect your recoil at all