for me it takes two forms which can be easily conflated.  both sets of sensations are very warm and pleasant for me and centered around the belly and the heart.

the easiest and earliest have to do with doing metta towards those i love most on a human level.  in the times i was doing a lot of metta it was possible for me to feel almost completely free of the personal benefit component of metta and to reach a state where the sensations around my heart shone with a bright clear desire to benefit others exclusively.

it was finer, clearer and spacious and i would now compare it to the higher jhanas.