What will happened if I use mix_precision but don't add with accelerator.autocast():

As stated in the title, I am a beginner of accelerate, I found that if I set Accelerator(mixed_precision="fp16"), the code runs normally, but when I turn off the flag(don't set mixed_precision) the code raise OOM error. I wonder what will happened if I only set the flag and don't add accelerator.autocast()