Rashford has suffered lost of form last 2 years due to weakness in his mentality. Rangnick stated that he trained very well but can't transfer the form into the match. His confidence hit rock bottom last season and almost looked like he was going the path of Danny Welback.
ETH came in and worked with him more on his mentality side rather than physical. Rashford has always been very fit.
I think the celebration is to remind him to keep believing and develop his mental strength. He can be the 40 goals a season striker if he focus and believe.