A yoke is a kind of harness (the bit around the neck) that oxen and horses wear when pulling carts and ploughs. If you're yoked - you're a working horse or ox! So throw off the yoke and run free! Shackles are leg-irons and chains - used to secure animals and slaves. Again, break free! So you can use the expressions if someone is, for example, feeling tied into a job or marriage they do not like. "Throw off the yoke! Divorce her!" or "I'm finally throwing off the shackles! I'm retiring!"