"So, how do you perform a soleus pushup?
In brief, while seated with feet flat on the floor and muscles relaxed, the heel rises while the front of the foot stays put. When the heel gets to the top of its range of motion, the foot is passively released to come back down. The aim is to simultaneously shorten the calf muscle while the soleus is naturally activated by its motor neurons."
I think that gives the reader enough to replicate the Soleus Pushup - perhaps an indication on where the effort/force is to be emphasized/felt would help.
Looking up "heel lift", the Soleus Pushup reads/sounds a lot like a seated heel lift. see https://www.livestrong.com/article/137423-heel-lift-exercise...