I'm not sure of the background behind it personally, but I remember a bit of a fiction and accompanying artwork in the Imperial sourcebook for the Mutant Chronicles rpg. The piece describes a "grav-shute" being used and the artwork shows it to be a backpack contraption consisting of fuel tanks and retro rockets used to slow descent.
This is quite possibly the idea behind giving Storm Trenchers para-deploy; they use these grav-shutes to air drop from a great hieght on the battlefield, controlling their descent by firing the rockets.
That's what I think.
Jase