they put their hands---(back.up,.in front,down)

they put their hands down.
他们放下了他们的手.
put down
v.
放下,
如果后面是and got ready to shake hands with them的话
就应该选put up了,put up是“举起,抬起”的意思.
整句话意思是：他们抬起他们的手准备和别人握手.