This idiom originated from racing - when the rubber hits the road is when the automobile starts moving forward.
It means: "when you get to the action", or "when you get busy", "when it gets serious",...
A condom comes rolled up in a package. Just place it on the tip of your erect penis and unroll it. It is important to put it on the right side up otherwise it won't unroll. If it helps you can unroll...