It's perfectly manageable to whip finish a fly with fingers alone, however many tyers discover using a specially made device advantageous as it will make tighter loops and stop the threads unravellng when used properly.
Types vary but normally a good number whip finish devices contain a straight handle with a hook at one end along with frequently a sprung wire arm.
The Rotary whip finish tool is a fast and easy tool to use.
It can be used by both left and right handed fly dressers.
Just that bit easier to use than the fixed head whip finisher - but give yourself time to get used to it.
Available in a neat standard or longer reach version.