From now until 15 Nov 2018, you can earn an additional bonus 4 mpd on top of the usual base earn rate when you use your American Express KrisFlyer co-brand cards on Apple Pay, Samsung Pay or Android Pay.
The bonus miles that you can earn is capped at 1200 per registered card, meaning that you will reach the promotion cap with S$300 of spending.
Do note you must be the first 20000 cardmembers to register for this promotion to be eligible for the bonus miles. I had some difficulty registering for the promotion via the Amex app earlier, but had it rectified after a couple of phone calls with them. Full terms and conditions can be found here.