Straight into the jar. It's a one-way valve. When you squeeze the brake lever, the brake fluid can flow out but air cannot flow back in. Super simple. So instead of using a vacuum pump to suck, I use the brake handle and the brake foot pedal to pump. There is a procedure that you need to follow because there are multiple paths that brake fluid can take. There are many articles here that describe the process for bleeding but that tool will make it simple.Straight into the jar is how you'd normally use it.