Take two 1/4" plugs, one TS and one TRS, and lay them side by side. Now think about the jack that the TRS will plug into-it will have three contacts that match up to the tip, ring and sleeve. Now look at the TS. You'll see that the sleeve is longer, so that it covers the same area as the ring and sleeve combined on the TRS. So on your TRS jack, both the ring and sleeve contacts will be in contact with the sleeve of the TS plug. Just as a side note, if you don't plug the TS all the way in (at both ends), you can get the tip of the TS to contact the sleeve. Then, the switch that was always engaged when the TS is all the way in will work normally, but the other one will be always off. This last bit of course isn't a practical suggestion, just something to help you understand why the TS cord won't work.