Keeps constant tension on the muscle so more blood gets pushed into it i suppose.
Bicep (for example) is a very simple muscle and you can easily keep it flex'd even if you are performing full ROM reps. I would also claim that you get more out of your workout when performing full reps.
I thought Phil's reps were ok. He went to the top of the active ROM on each rep, but instead of straightening his arm at the bottom he did the, as Comin-up said, the constant tension method. Nothing wrong with that as he was covering essentially a full ROM. Rob wasn't though.
Phils curls were great, watching rob do 4 reps i closed the video. looked hybrid front raise and curl with poor ROM as already discussed. They are just curls though obviously other things are giving rob his big arms.
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Who cares if they do full reps - it obvoiusly works pretty well for 'em. When it comes to arms a lot of people feel a better pump/contraction when they don't do a stright full rep - I do that to and arms are one of my better bodyparts.