for big bench, arch as much and as hard as you can… I got my bench from 130 at 75kg class to 250 at 90kg class in less than 2 years, same shirt, 10kg heavier, 120kg bigger bench, just by practising arch…
as far as muscle isolation, you’ll get a lot less with an arch… so you’ll need more assistance work sets to keep muscle stronger… the way I think about it, when I prepare for the meet, first I do movements then I do muscles…
I barely grew while I was training arched bench… I could press more and more, and keep adding weight to my bench, but muscle size stayed the same, sometime even dropped…
now, I stopped flet benching for 2 weeks and I do inclines only… I’m more into muscle isolation now, to feel the muscles being worked, not weight being lifted, and on FLAT bench, by default my back arches, so I just use inclines, DB’s etc.
but for big number ARCH as hard as possible, bar to the belly, wide grip (will take some time to get used to it if you havent used it), elbows in on the way down, and out on the way UP, shoulders pulled together at all times during the lift (you’ll need extra trap work for this maybe)… stretch the quads and hips… so you can get your legs further back…
lower back stretches are not as needed, I never did them, because back is actually in FLEXED position, not stretched position during an arch, and quads are stretched for example…
make sure you understand every aspect of big arching to get it to maximum, just laying on the bar and trying to arch more and more will get you only so far… sometimes you’ll need special work…
on my videos, on this site, you can see one old video (2 years old) of me practising and showing the arch for RAW bench, technique differs a little bit, cause you cant use your hands to push away from the rack, but I developed both techniques very well, as well as feet flat on the floor or toes only…
video was only made for myself so I can see how it looks from outside, but I posted it here, maybe it can help someone…
gavra