This thread is all over the map.
Default shutter on R is EFCS or electronic front curtain shutter (Silent LV shoot – enabled).
EFCS avoids shutter shock similar to using mirror lockup on a DSLR. The front curtain on a mirrorless camera carries most of the vibration, so EFCS gives you the best of both worlds by using electronic front curtain with a mechanical rear curtain (thereby avoiding the possible rolling shutter with full electronic/silent).
Where EFCS is problematic is when shooting wide aperture at shutter speeds over 1/500 of a second. In such cases, you may see bokeh artifacts (sharp edges to blur areas), reduced bokeh (f/1.2 blur may appear as f/1.4 blur), or partial reduction in bokeh (ex: half of frame appears normal at f/1.2 but other half has reduced bokeh).
Full mechanical shutter (Silent LV shoot – disabled) should be used over 1/500 sec if you're shooting at wide apertures in bright light. Otherwise, keep it on EFCS. This is the mode where you will hear what sounds like two exposures being taken – both front and rear curtains are moving. The slower the shutter speed the more pronounced this auditory effect will be. At fast shutter speeds it sounds close to EFCS.
Canon should not have named these things so cryptically they way they would on a consumer-oriented Rebel. Call them what they are, Canon. I suspect they may change the labeling with the coming high-res and 1D-class R bodies.
Canon also could help us out by doing what Fujifilm already does – giving the option for "extended" modes where you can automatically switch from EFCS to mechanical shutter over 1/600 sec. with the additional option to move to full electronic/silent shutter once the exposure requires a faster shutter speed than the mechanical shutter can reach (either 1/4000 or 1/8000 sec. depending on the model.). This full extended mode automatically moves from EFCS to mechanical to electronic allowing the photographer to avoid EFCS bokeh artifacts or blow the exposure when exceeding 1/4000 or 1/8000 second in bright light.