Using a raised voice - GO DEEEP! - when the vagina is propped up and ready for insertion. This catch phrase is often helpful for increasing focus for the task at hand. The penis is inserted at a medium pace, ensuring full length penetration. The first stroke often sets the mood for the remainder of the session, so it is cruicial to maintain direction and speed, and ensure maximum depth into the vaginal cavity.
Alternatively, the partner (or receiver) may request the giver to 'go deep', which following guidelines of the first context, also sets a great mood.
In a baby making context. To go deep is to ensure the ejaculate from the male testes is deposited as close to the uterus as possible. This ensures the semen has the least distance to travel through the cervix, as the spermicide contained within the vaginal walls battles with the foreign invaders (similar to the asian invasion occuring in Australia... you know who you are!).
Trent: inserts penis into anal cavity "Oh yeah Seanny baby... my cocks so deep, i can feel your intestines"
Sean: "Your not that large baby. I mean, ive taken bigger."
>>> Dave: "GO DEEEP!"
Dave: inserts penis into Nancy's vagina acurately
Nancy: "Oh yeah... this will be an intense fuck"
>>> Rose: "make sure you go deep this time, im sick of fucking you to make a baby"
Barry: "alright im gonna cum... its in as far as it goes... OHHHH OHHHH!! ahhhhh yeah!"