What does the spark feel like?

The spark is a certain feeling of sudden heat--even alarm--that seem to start in the heart region (literally--I feel it underneath my sternum somewhere). It quickly migrates to the netherparts (i.e. those covered by my loincloth).

What does I feel a spark between us mean?

It can be described in the terms of mutual feelings - a connection, a bond or common feeling between two people, or as a chemical process - [it] stimulates love or sexual attraction brain chemicals are definitely involved.