It's a kind of ellipsis (the sign for ellipsis is "..."), to understand this phrase you must add a fitting word. Když jinak nedáte ... = If otherwise you don't give ... or maybe If otherwise you won't give ... The speaker did not finish the sentence. This incomplete sentence is common in a conversation, but rare in the written texts and nonexistent in the official documents. Examples of more complete sentences: Když (mi, nám) jinak nedáte (svůj) souhlas. = If otherwise you won't give me/us your permission. Když mi jinak nedáte (své) požehnání. = If otherwise you won't give me your blessing. Když mi jinak nedáte peníze. = If otherwise you won't give me your money. The "if you insist" is not word-for-word translation.
I disagree with Troll - když jinak nedáte is a complete sentence and it definitely means if you insist. As it is a reluctant positive answer to a previous statement, all you need to know is the context of the conversation. E.g. "Půjdu s vámi, jen když si budu moci vzít s sebou svého psa." (I will go with you only if I can take my dog with me.) "Když jinak nedáte..." (If you insist...)
