If you board the train (tap in) before 7.45am, up to $0.50 is deducted from the train's portion of the fare.
That means if you start your journey with a train trip, the fare will be reduced by $0.50. This is a guaranteed because the minimum fare upon starting a trip is $1.09 (after fare increases). You will get the full discount, so it'll be $0.59.
HOWEVER... if you start the trip with a BUS journey, the minimum starting fare will now be tied to the bus journey. If you take a transfer trip afterwards, the $0.50 will be deducted from the train's leg, however much it costs. If you travel far enough on the train, it could still be more than $0.50.
So, there are some scenarios where you might want to walk to the MRT instead of taking a bus there, to save a bit more money.
Check out the examples here if you didn't get that.