From what I have been told, the taxi drivers have their own collective little "unions" that offer these discount fares. It's just a matter of who you call, and who provides the service.
There a number you can call for discount fare taxis. Its 61100871.
They offer a 20% discount on the metered fare during the day and peak hours, and a 30% discount on the metered fare during the evening-nights/off-peak hours and longer distances (e.g. to/from HK Airport, New Territories, etc..)
All tunnel tolls are charged one-way only. Minimum fare must be over HK$50. Calls must be made at least a half-hour in advance, and sorry, they only speak Chinese. Get somebody to arrange one for you if you can't understand.
