You press A (to activate auto mode) when her hands are in position for a handjob. You can still move her head using your mouse, but her hands should work automatically. You can also set his arm to free, if you want it to look like the girl is doing everything herself.
It doesn't look quite right with a double-handed handjob if you try to get her to deepthroat in this automatic mode (which you can't actually do, as the hands are in the way).