In this clip you see Roan and her Nana Pam (grandmother) playing. My objective was to get Roan to move from sitting into kneeling. My legs provide support for her attempts to reach toys which I present just out of reach.
Playing when kneeling at a low block provides opportunities for exploring kneeling as a position, transferring weight, and strengthening the arms in readiness for crawling.
Once an infant has mastered the basics of crawling, it is a good idea to provide him/her with opportunities for crawling up, onto and over different surfaces.
In this video clip you see the impact of the position of the hips on the infants ability to move from sitting to kneeling at a low block. 
Once an infant has mastered the basics of crawling, it is a good idea to provide him/her with opportunities for crawling up, onto and over different surfaces.