And you can't spit nigga, so you obviously must swallow
Apart from the obvious physical spit and swallow, there is a nice double meaning: to spit = to rap like it's an easy thing to do ("to spit rhymes out"). So free translation is: "when you want to battle and can't rap, then you get to suck a cock"
Soleone
It is important to note that this is not a logically valid personal attack, as Mr. Ace commits fallacy of equivocation by implicitly substituting different meanings of "spit" and "swallow"
Epicurus