Spring Loaded? Looks like I was misinformed, my visual scan of the belt looked like it was 'adjustable' by wrench only, just like all my prior vehicles. I'd also swear I read somewhere it needed manual adjustments. But, here are a couple FJ Forum posts that show it's spring loaded.
I messed up a belt install once, the grooves were off by one and it shredded after about 20 miles. The strands in the belt got tangled up in my steering pump. Big mess. Just make sure it's seated properly on all pulleys. The tension is automatic from the spring tensioner.