some people change their toss, iīve even seen people change their stance, others stay sideways on slices and kicks. had a doubles opponent recently who changed his position when serving wide or down the middle
had an opponent once who would always stare at me for an extra second, before hitting flat firsts
for me itīs like this. i feel that even very good players sometimes lack the ability or the understanding how to read or evaluate their opponent.
to give you a direct answer to your direct question
i donīt focus my eyes as much as i watch the player as a whole