Learn German: Wer das Geld verliert, verlässt den Tisch, den Kopf gesenkt. - Whoever loses the money leaves the table, head lowered.

This is the Absolute Accusative (Absoluter Akkusativ). German uses this structure—a noun in the accusative followed by a participle or adjective—to describe a posture or physical circumstance that accompanies the main action. It lets you skip the preposition entirely for a more elegant or literary style.

Why does the verb verlässt come right after the comma? Shouldn't there be a subject?

The entire first clause, Wer das Geld verliert, acts as the subject of the main sentence and occupies Position 1. Because the conjugated verb in a main clause must take Position 2, verlässt immediately follows the comma.

Is it possible to insert a pronoun like der before verlässt?

Yes! You will often see sentences like Wer das Geld verliert, der verlässt den Tisch. Adding the demonstrative pronoun der (he/that person) to refer back to the first clause is very common, but leaving it out as this sentence does is also perfectly fine and sounds a bit more concise.

Could we use the present participle senkend (lowering) instead of the past participle gesenkt (lowered)?

You could, but it changes the meaning slightly. den Kopf gesenkt describes a static posture: the head is already down while the person is leaving. den Kopf senkend would mean the person is actively in the process of lowering their head as they walk away.