Today I worked with a lady during her first BJJ class.  I first worked with her during the technique instruction portion of class.  As she became comfortable with the Mount maintenance and stabilization drills we were working, I gradually increased the amount of force I used to attempt to escape or unbalance her.  She did great.  

Later, while sparring, my instructor asked me to roll with her during one of the rounds.  Despite having demonstrated earlier that some force/strength would be in play, I had a tough time getting her to exert any force on me.

Despite the fact that there's little love out there for starting on your knees in BJJ, my BJJ instructor, Jose Portillo, suggested that rather than starting from a sitting position with new folks (I'm reluctant to start standing until I've got some idea of my partners skill, athleticism, and aggression) we start on our knees instead.  I can still basically let them have the top position, but to take it they need to exert at least a little force to get there, and they'll probably intuitively understand that they need to at least topple me over.

This is the first time I've heard a compelling reason to start on your knees; I usually avoid it in favor of the more practical (standing) or injury-preventing (one person sitting and attempting to pull guard) options.  I'll be interested to see how well this works, next time I have a similar opportunity.