Are you slouching? If that is the case then your trainer might be telling you to arch your back so you have a straighter back? I don't think it's the best way to word it but I could see how it could work.
Maybe when you think you have a straight back, you're really slouching/have rounded shoulders. So, when your instructor tells you to arch you're back and you do, it's really just straight.
Kind of like when my trainer used to tell some students in my lesson to canter - they'd go really slow over the fences. So, she'd tell them to GALLOP! And they would go faster, not galloping, but a more forward pace for jumping.