Quoting the Youtube description which pretty much sums it up:
Youtube wrote: I've become slightly obsessed with transporting animals to their death using minecarts recently. Here is another setup that drops them into a hole containing saws.
The idea is that using Block Dispensers, the following sequence of operations is carried out:
1. The cart is picked up.
2. Some rails are picked up.
3. The blocks beneath the rails are picked up, revealing the deep shaft.
4. The blocks are replaced, followed by the rails and minecart *except* the rail that was right behind the cart when it was picked up
5a. In that place, another solid block is placed which makes the minecart move away.
5b. The solid block is picked up and replaced by the rail again.
Jumping animals can escape, but this could be easily fixed with more BD's and longer delays. They also seem to be able to get stuck sideways, but the same solution should work there.
The main parts of the wiring is shown. What is not shown is the redstone wires leading to the BD's on the other side, but that is exactly that, just wires.
Some of the BD's could be replaced by sticky pistons, but I always have trouble with wiring those since they are activated by neighbouring powered blocks too.
Also shown briefly in the video is the holding loop for (occupied) minecarts where I did some experiments with ensuring that carts move in one specific direction and a way to move them onto another track on demand.