After watching all of the trainer Mega Evolution animations here, I have come to a simple solution — she does in fact press her hands to her Key Stone, before she spreads her arms.
Here, you can see that her Key Stone is glowing under her hands, and the light is therefore only revealed when she moves them away from her chest. You can see in the video that many other trainers, such as Vinny, also do not hold their Key Stones the whole time ( he, for example, presses his other hand to it before pushing his palm with the Key Stone outwards ). In all of these cases, the light from the Key Stone bursts out as the trainer stops touching it.
I would love to say that there is a lore reason behind this, but I am more inclined to believe that the goal was to implement unique animations for every character, and it just so happened that Emma’s animation is a bit ambiguous as to whether she actually touches her Key Stone or not due to it being covered with her hands. It is likely that she does, though.
