A withy is a supple green tree branch that has been twisted into a flexible binding - it then becomes an invaluable tool for binding things together.
Here's a woodland equipment and drying rack I made of hazel poles bound together with hazel withies:
To make a withy you need to twist and twist a green (living) branch, and then twist it some more! This will pop the wood fibres apart, and making a thick branch into many thin fibres introduces flexibility which means you can wrap and tie things up with it.
You can then split the fibres into bundles depending how thick and strong you want them - you could split right down to fibres a few mm thick, or keep bundles that comprise many fibres and are quite strong.
Here's a trout held in a hazel pole tied with a withy:
When selecting a branch to make into a withy, hazel is a good choice because you can often find many straight, knot free, fairly thin branches. Other woods could work too, like sycamore, alder, ash or willow. Just make sure to avoid knots which make the branch tougher to twist and hold the fibre together.
Choose a branch about an inch in diameter and as long as you want your binding (finding a good long 5 or 6 foot length is good). The thicker it is, the tougher it will be to twist. Strip the side branches off by just snapping them or pulling them off by hand, or if they are tough, slice them off with a knife.
Keep the branch attached to the trunk! If you remove the branch first it'll be really difficult to twist because the free end will keep spinning around. It is possible to tie it around a tree or stand on it to secure it, but it's much easier if you leave it attached whilst twisting then cut it off at the end of the process.
Twist the branch until you can hear a pop as the fibres separate. You can bend the end branch to create a sort of crank handle to give you more leverage to twist.
Twist until the branch kinks, it has then reached the limit of its ability to twist up and you need to move down onto the next section of branch, below the kink. Keep twisting and moving down until the whole length of the branch is twisted. Go back and check each section for suppleness - you'll most likely find stiff spots that need to be twisted to make them really supple.
Finally, cut the base of the branch to free the withy.
The benefit of withies is that they are quick to make and extremely strong under tension (pulling force). Unfortunately they're not very robust under fiction (being rubbed), so break apart quickly in that scenario. Another huge benefit is that many bush cordage techniques use either green plant stems like nettles or brambles, or the inner bark of a tree - both of which are only readily available in spring and summer since tree bark does not usually separate in the winter, not do tall green plants grow.
After splitting the fibres you can also plait them as you would plait hair; laying the outer strands over the middle strand. This increases strength - here's a withy I plaited in this way:
In summary, I'd highly recommend getting out and trying this technique - it used to be used commonly by countryfolk to tie bundles of firewood or hedge laying stakes. The fibres are strong, biodegradable and free! Just make sure to follow sustainable harvesting techniques by not taking more than 10% of any branches you find.