I am looking for this tool and can’t find it anywhere. JC Whitney no longer seems to carry it. It was called an Instant Fan Belt Tensioner. I spotted it here.
It’s used to force two pulleys on the face of an engine apart to tighten the belt around them. Modern engines have tightener devices built in. Older engines don’t. Marine engines like mine (and the gent’s on the link above) don’t. Yes you can use a long screwdriver to lever it out but it is a two-handed job that ends in skinned knuckles, cursing and bent screwdrivers.