If you tie a figure-8 knot while rock climbing and fall a few times (especially if they're dynamic lead falls), the knots will become so difficult to untie that you sometimes have to use a carabiner to help you untangle it.
Are there any techniques one can use to make this easier?
The figure-8 knot is characteristically hard to untie after falls. What many climbers do is, once the knot is done, to come back with the end of the rope inside the last bight - this is a sort of Yosemite finish. It makes the knot easier to untie, since after removing the extra strand, a lot of room is left for the rope to be untied (picture here). Andy Kirkpatrick, in his book "1001 Climbing Tips", says that using a rock or an aid-climbing hammer to smash the knot can make it easier to untie, too.
My personal opinion is: a knot that is so hard to be untied is a bad knot. The figure-8 was OK when climbers very rarely fell, and ropes where made out of hemp and whatnot. Nowadays, with sport climbing and falls becoming so normal, the age of such antique knots has come to an end. The double bowline or, better yet, the end-bound single bowline (EBSB), are easy to tie and inspect and can be untied with minimum effort. I have used the EBSB for the past 3 years both in multipitch and sport climbing (convincing a dozen other climbers to use it in the meantime), and I'm extremely happy with it.
I notice the answers to this question have not been updated with more recent discussion of this issue inspired by the video "Why Figure 8 knot is NOT hard to untie!". The Figure-8 knot can be hard to untie after a fall if the load line is the upper of the two lines around the bight, but it is easier to untie if the load line is the bottom of the two lines. The problem with load line on top is that it squishes the whole knot after a fall. For just the essentials about the two versions of the knot, start the video at 13m 24s.
For some nice colour coded images of the two versions, check out "A better way to tie the figure-8?"
I’ve experimented with this a bit. We tried pulling the knot really tight or leaving it really loose. It turned out that pulling it relatively tight (but not too tight) works best.
It also makes a huge difference how you start untying. Start with wiggling/folding the parallel loops where the end of the rope comes out. Once you get a tiny bit of room you can undo the knot.
It’s still hard and finicky, which is why I’m mostly using the bowline on a bight knot (it’s the alternative common knot aside from the figure eight, here in the German speaking area) now. It’s a bit harder to tie at first but always easy to untie after a fall.
This is what a marlin spike is for. https://en.wikipedia.org/wiki/Marlinspike
It's not special purpose just for figure 8 knots, but is a general tool for tight knots.
Dates back to the age of sail, when natural cordage would get pulled tight then got wet and would swell.
