Hmmm... when do I get the healing hands and the animals all being friendly to me? More seriously... when do I get martyred... Its nice to know these things in advance so I can buff up my pecs for when the Italian Renaissance artists are sketching me for the half naked martyr stained glass windows
With reference to some of the comments about it not working in practise, I agree. No system is ever perfect and will never be 100% effective in practise and when you add endorphins and adrenaline and testosterone into the mix you amp up that liklihood of forgetting what the signal is. This is why you always have back ups. I agree with the points about safe signals being sometimes a goad to push harder. I know from my own experience that I have gone further than I would normally because I knew I had a safe word. Having it made me not want to use it to end the session. Odd reversal... Ths is why you need the Dom/me keeping a careful eye as well.
With regards to what to use I think the method would depend on the couple involved and the activity. Some methods of silent safe wording will not work for some couples or for some activities - some of the reasons are already discussed above. You do need to think about what you intend to use and whether it is practical or not.