When all three forms of austerity — bodily, verbal, and mental — are practiced with deep faith, by disciplined people who seek no reward, Krishna calls that sattvic tapas.
Two marks distinguish sattvic austerity from every other kind. First, there is genuine and deep faith behind it. Second, there is zero expectation of personal gain. The person practices austerity not for fame, not for heaven, but purely for inner purification and devotion.
A river flows without asking for anything in return. It simply does what it is meant to do, giving water to everyone along its banks. A sattvic ascetic carries that same spirit — doing what is right, steadily, without keeping accounts.