What you seem to be forgetting is that this technique pushes one's body beyond our limits. Pain exists to "motivate the individual to withdraw from damaging situations, to protect a damaged body part while it heals, and to avoid similar experiences in the future"; in a sense, it's a positive thing as it allows us a signal to know we're injured and need to heal/rest. By taking away the pain we feel, our body doesn't get a rest, and we can end up further injuring body parts which should be producing pain: letting us know that moving them is a bad idea. For example, if you used the Eight Inner Gates (a lower number), you destroy your muscles, but you can still move them, though this causes extreme pain. Using The Drain, you don't feel the pain, but you continue to destroy your muscles by moving them beyond their now-passed limit.
Another drawback to The Drain is the fact that it's not actually healing, despite the fact that it feels like you're healed. For example, someone breaks your leg. You physically cannot stand on your leg - it's not just the pain, it's the fact that your bone isn't connected, and as such can't support your weight. By using The Drain, you don't feel the pain, which means when you go to stand on your leg, which feels fine, you can't and fall over - it's a memory game to remember what you physically can and cannot do, rather than having the pain there to let you know if you happen to forget. Also, if you're bleeding out, it doesn't stop the bleeding, only the pain - you can still die from blood loss (as an example).
It is passive, there is no technique for it, but you do have to state in your post that you cause the Keigoku to flow back towards yourself and touch your body to "deposit" the "negative pain" to firm up your mind and make you forget your pain.
Now, for your next post, please go over how you believe The Drain works: Your view on the mechanics behind it. Don't disappoint <.<