The BT-2 4th-Hand Brake Tool is a ratcheting wrench that allows you to pull cable and hold it in place as you make brake and derailleur cable tension adjustments.
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Great tool - wish I had it sooner. Needed it to help get good cable tension when setting up a mechanical front derailleur. It is the right tool for the job - much nicer to your cables than a pair of pliers.
Because of the changing of the industry this is probably becoming a less and less needed tool but is awesome for tuning up older bikes. It is nice for certain dropper cable install situations, or helping cable housing ends seat before you set them up. This doesn't crimp the cable too bad but it would be nice if they put a little groove so it cupped the cable and didn't crimp it at all, just held it tight. Don't think park tool care's about my opinion about it but thought I'd mention it anyway.
I’ll echo what others have said about this tool: it’s not an absolute necessity, but it can be really useful.
For me it really shines when setting really old front derailleur cables that don’t have barrel adjusters. The tool allows finer control during initial tensioning than using a pair of pliers. Old pro wrenches will scoff and say it can be done without this tool. And they’re right. But any tool that makes a job easier means the job goes faster which means you’re out on the road/trail faster, too.
If you’re just getting into cabling your bike yourself, invest in a good set of cutters first. This tool can come later.