A gymnastics performer is in mid-pose as she completes her jump.
A gymnast is completing her jump.
"A gymnast is completing her jump." is a rephrasing of "A gymnastics performer is in mid-pose as she completes her jump."
"Is completing her jump" is a restatement of "as she completes her jump".
As she completes her jump in mid-pose, implies that she is in process of completing it.