How are Prasangika and Svātantrika different?

I was refreshing myself with some stuff on Madhyamaka. I don't understand how the difference between Prasangika and Svātantrika can be svabhava. How can svabhava exist without changing anything else about conventional entities or ultimate reality? So what other claims does this entail about conventional entities by Svātantrika? Or does Prasangika show their ultimate reality cannot be empty?