A tag is a free invoke. A tag therefore can do anything an invoke can. An invoke can invoke for effect. An invoke for effect can target an aspect other than one directly possessed by the invoker. An invoke for effect targeting an external aspect found on another character can thereby trigger a compel as the effect it's being "for effect" invoked. Full stop: all invoking actions at this point have concluded. Since a compel has been triggered, however, the compel machinery remains in motion. Compels are negotiated and run between the GM and the target. It resolves however it resolves. Sometimes the target accepts it. Sometimes the target rejects it. Sometimes the target and GM determine, y'know what? This is weak sauce and doesnt count and the compel doesn't carry through. This is the last I'm saying on the topic -- I've had like five different people email me about this over the last week and would've hoped more folks were pooling their notes on this. I'm a bit sick of the question, a bit sick of answering it, and would appreciate it if folks asking for some official-word advice (a concept I don't much love) would maybe not argue with me when I come around to give it.