Set your shooting aperture, depress the aperture preview button then de-couple your EF lens and twist it 1/3rd away from the lock.
Good tip! Why didn't I ever think of that?
Is this different than shooting full manual? Does decoupling save battery power?
It will save battery power I suppose but thats not the main reason.
Even if you shoot on full manual the iris is stopping down each time, and it may not be exact every single time, so you get a slight exposure step, can be 1/10th of a stop, but this will be visible on your timelapse.
By de-coupling the lens with the aperture set as described the aperture is closed and thats it. It will be the same for each shot because it is effectively locked.
Another reason is that a 10s timelapse may be 250 or 300 shots. Thats 250 or 300 opening and closings on your aperture diaphragm and electronics that your lens could probably do without.
NB> If you use adapted manual lenses or zeiss cine or samyangs with a mechanical iris then this won't be an issue.