Mercy's strange vertical boost - how does it work?

Solution 1:

You might be referring to how pressing the jump button during Guardian Angel will cancel the ability while retaining your momentum. If you happened to press jump while moving up at the time, this would have given you a vertical boost. As a result, doing it consistently requires a teammate to be above you.

Solution 2:

This is the slingshot effect on Mercy combined with the way the player model works with teammates. The way it works from testing is that when you start the guardian angel spell you get the sudden burst of velocity, and when you use guardian angel, you bounce off the player model and with the velocity get shot up into the air and get a lot of vertical height.

It works better when the teammate you use guardian angel on jumps and you use it at the peak of their jump.