Smacking it properly takes the place of hoping the belt will hold in many cases yet that doesn't always work. Sometimes you can put a ratchet on the tensioner with a 15mm socket and REVERSE the pull to hold the belt TIGHTER to help out.

I use the Kent-Moore tools....LOL.

Aftermarket tool companies make sets for these fans (and others) too if you'll be doing more than one. There is even an air hammer bit with the wrench on the end you can use without having to hold the water pump pulley.