Some editors think it best to get a feel for the correct point for the In and Out of each clip while the clip is playing. As it is quicker to release a key than press a key, in Final Cut, the In point is set when you release the ‘I’ key and the Out when you release the ‘O’ key.
Whether the video is playing forwards or backwards (J key or Shift-space), you can set these points without tapping the keys.
Alternatively, you can do the same by holding your mouse down on the Mark In or Mark Out buttons until you are happy.
Again, this is from the manual, but when I last looked at this section, it said tapping I or O was the way to mark in and out.