When I first taught thunder, It was at a clinic and they had us put pressure on the lead line down to their chest and when they even shifted their body in the direction of attempting to go backwards, praise them much. Then you get them to take or even just lift one foot, big praises. And so on.
Another way I've seen people do it is with a dressage whip and tap (not slap) the front of the legs and/or chest start small and then go harder (but constistant, not beating) and as well putting pressure on the lead line down into the chest until they make the slightest movement backwards...then reward (same as above).
As with the top one, I've seen some people hold down the lead and poke with finger into the chest asking for the back.
Remember to give your "movement" cue, like a click or kiss asking back.
And then when you get it down on the ground asking for it in the saddle can be a bit more difficult and confusing for them, but you want to keep your hands lower, pulling into your belly button and slide your hands down the reigns, and from what I'm told, you do have to ask for a lot of contact. As well as cueing with legs and asking for the back. Hope that helps!