was taught that when you send Distant Reiki Healing to someone, you never tell the energy to do this or that, as it could be construed as trying to manipulate the energy to do so.

Distant Reiki Healing is  a process whereby a Reiki Practitioner (Level II and above typically) sends the energy of Reiki to someone, or something, in a specific ritual we perform to do so.  We can "send" Reiki to anyone or anything on the planet, past, present or future, for any reason under the sun.  It's sort of like praying.
In the past, I have always instructed my students to never state a desire for a specific outcome when sending the energy, to just intend that it flow to the person or situation in whatever way would be for their highest and best good.

However, I am now rethinking that premise. I also taught that we have the person we are performing an "in person" Reiki session on, set a mental intention for what they'd like the Reiki energy to do for them, at the beginning of a Reiki session, meaning we are not privy to that information.

Then I became a Level I student of Healing Touch, another energetic healing system created by a nurse and very acceptable to the Western Medicine community, much more so than Reiki, at this time. In Healing Touch we set the intention with our client verbally for the session, before we begin the session, and I found this to be very powerful.

I began doing this with my Reiki clients, and found that it made for a much more powerful session, and that things were happening that I didn't think were conceivable during and after a session.

I had an insight yesterday afternoon that if we can set an intention out loud and together with our  "in person" clients, then why the heck couldn't we do the same when sending Distant Reiki?

So I am experimenting with it now with a student of mine (we are sending each other Reiki with intention for the healing of several different conditions we each have), and have been incorporating this into the other Distant Reiki Healing sessions I do on a regular basis (I host two Distant Reiki Healing groups, and belong to several more).

I invite all those who are Reiki to consider doing this and see if they don't find that there is no problem with it, that it will actually enhance their Distant Reiki Healing practice, and begin spreading the word. 
As in all things, even though we ask that the energy do this or that, we might also want to add that whatever is for the persons highest and best good be the outcome.  We never know what the Universe/Spirit has in mind for someone, which maybe why they are experiencing what they are experiencing, so we do not want to interfere with their process, or take on that Karma.