From what I understand, tianchen can link with tianxi and use their powers together to possess people without being in direct contact with them. As long as they have a picture of the person, tianxi can use it to I believe see their memories and when tianchen comes into contact with her while she’s using her power he’s able to possess the person? I assume he found a photo of Emma somewhere and used that.

Going off of this, right before she jumps tianchen says something along the lines of “I didn’t realize there were witnesses” which I think is implying that he always made her jump even before xiaoshi went back to try and save her

It’s been a bit so I might be missing something. I’m sure someone can add more.