To get to that position, by himself, he would've had to stand behind the rack, unrack the bar and walk to the end of the bench, barefoot, carrying 315lbs. To put the bar back, he would have to walk backwards with the weight.
You just rest the bar on the bench,bend at the knees and start from the bottom,thats how we all used to do it.Most i went up to was 2 and a half plates but one guy at our gym used to use 3 plates no problem.Your lower back strength builds up over time and i can`t remember anyone having back problems.its correct what Coach said,if you`re used to machines then going to free weight will always find out the weakest link in your body.Its the same if i use a machine on squats,when i go back to a barbell its my lower and middle area of the back that gives out first.A chain is only as strong as its weakest link.