Timing, like many things, is most easily explained by looking at what happens when things go wrong. Bad timing may mean you are late. If you are late, you speed up, get flustered, run through a red light and get a ticket. If you are early, you show up and the library doesn’t open for another hour but you have to go to work in a half hour so you can’t get that book you’ve got on hold until tomorrow but the library is going to ship it back to the central stacks tonight so you’ll have to put it back on hold.
Bad timing, in other words, means you’re either early or late. Good timing means you executed the technique at the correct moment and it worked. Good timing happens more often when you are relaxed and focused on the task at hand. Timing is improved through practice. If you want better success at figuring out when to put on the arm bar, practice the arm bar a ton of times with a partner. Practice it first with no resistance and then gradually add a little bit of fight.
Practice, relax and focus. Good timing will come.