Well actually the back doesn't go anywhere. The impression of the back lifting is given by the contraction of the abs. This is achieved by a horse who can stretch, is supple and pushes down and out with their nose.
So the horse should be holding the bit down as if you were holding hands and with rythm they will engage those muscles.
If you look at when he's eating in his stall he will push down in the feed bucket (which I guess is mid hight) and lift the back. You have to achieve the same thing.
Note, curling the horse's head in, pulling it in, bending the poll of having the horse under the vertical will make it much harder instead of easier and with side effects.
Firm connection with the reins uphill and forward trot and elbows bent and leg!