How to get best timing accuracy?

What's the best technique to get an accurate time of a particular swim such as a 50,
100, or 200?

What triggers the start of timing? Is it the push of the button, the end of the trill, or when diving in after
the button push?

What triggers the stop of timing - is it the button push, or when the watch-bearing hand
hits the wall?

If one hits the button at 3 o'clock to start a session and lets it run continuously, how are the start and stops of
each lap determined?

Is there any way to get accuracy to better that 1/4 second?

The best way to get accurate timing is to control it yourself with the button presses. So, for the first swim interval you press Start at the beginning and Pause (bottom-left) at the end. For subsequent swim intervals, the interval starts when you press Pause to resume swimming, and ends when you press Pause again to rest. (You can tell the watch is Paused mode for resting because the display is reversed - white text on a dark background.)

The swim interval starts when you depress the button, not at the end of the trill. The trill is just to let you know that you did successfully press it and the watch is recording. The same thing is true at the end of the interval; the interval stops when you depress the Pause button. Since I usually rest in the shallow end of the pool. I usually get my finger on the button ready to press it, then drop down underwater and put my feet on the wall, then press the button right as I'm pushing off. (This manoeuver is a harder to describe than to do - I've seen other swimmers do it as well.) At the end I just try to press Pause as quickly after I get to the wall as possible. My guess is that I lose about 0.25 - 0.5 s doing this.

As you seem to have noticed, the watch will automatically detect rests if you don't pause the watch, but this will be LESS accurate than pressing the buttons. There is also some delay while the watch confirms you've stopped swimming, or have resumed swimming, so you will not know your splits in real-time in the pool. The watch also cannot detect very short rests. Finally, if you move your arm around a lot while you are resting, you risk affecting the accuracy of the subsequent swim interval. For these reasons, the recommendation is to always Pause the watch when resting.

I hope this answers all your questions.

Thanks for the answers! (I would have said that the pause button is at the 4:30 position rather than bottom-left)

Here's an idea that would help... To start, make it a race start sequence - press the start button, hear the trill as a "Take your marks", and then a beep 2 to 3 seconds later which indicates GO and starts timing!

Thanks for the answers! (I would have said that the pause button is at the 4:30 position rather than bottom-left)

Correct! I should have said bottom-right, or 4:30, as you say. Thanks for correcting me and for posting your idea for the "race start". I can see how that could be useful, perhaps as a customizable option so that those swimmers who aren't interested in timing with that level of accuracy wouldn't have to turn it on.