‘And she said, “See my maid Bilhah, go in to her that she may bear on my knees and I also may obtain children by her.” And she gave him Bilhah her handmaid as a wife and Jacob went in to her.'
“Go in to her.” A euphemism for sexual intercourse.
“Bear on my knees”. This confirms what we earlier saw with Sarah. When the maid bears a child she does it on her mistress's behalf. The child is Rachel's. But as Sarah's case demonstrated, the consequences were not always so simple when a blood child was later born. So the child does not rank fully with the true born unless fully accepted. It is to Jacob's credit that he does not differentiate between his sons. On the other hand in his case the slave children were not the firstborn. There is not the same rivalry as with Ishmael and Isaac.
The handmaids are subsidiary wives. There is no marriage contract, they but do the bidding of their mistresses. But their status and position improves.