The trick with the soleus is that its underneath the gastroc. And in many people's musculoskeletal structures, its entirely underneath the gastroc - meaning your trick won't help. Combine that with some compensatory activation of the gastroc during this movement and people won't be able to effectively train themselves to get the full effect of what the researchers are going for here - prolonged duration soleus activation.

I'm not saying they couldn't have done a better job explaining how to do this at home, but its a surprisingly difficult thing to explain to someone face-to-face when you're a personal trainer. Let alone when as a scientist when you only get a short blurb to convey information about your latest research study.