Donna stared into the Doctor's eyes. From her peripheral vision she saw his hands creep near her head; she knew iexactly/i what he intended to do - kill her in order to save her.
It made sense, in a way, she supposed. Death always seemed like the worst option in the world. And before her adventures with the Doctor she would have chosen life - any kind of life - over death. But, this was after the Doctor and death would have been better than the farce he was offering her - his best friend.
Donna felt his consciousness wash over her own - it was over powering and slightly painful. It made her gasp as her life with the Doctor flashed before her eyes - waving at fat, volcano day, doctor-donna-friends, we're not married, I'll have a salute, back of the neck, she belongs with us, harvey wallbanger, watch your hands, Lee - their life un-lived, a planet made of diamonds, turn left, there was something on your back, sorry for you loss, I'm sorry, so sorry - all of it was unraveled from her mind quickly taking the things she was most proud of with her.
She resisted. Pulled back from the Doctor. Fired anger and hatred and tried to scream to stop him and ... useless. She fell against him - her last act as Doctor-Donna - and knew he had done this ifor/i her rather than to her because despite the big brain and two hearts - that he never opened to anyone - he needed a moment of selfish salvation.
A moment when he could say: iI saved her/i instead of iI killed her, too./i
And she could understand that because she was his best friend and they had the best of times. So,she let him win the battle because he would never win the war.
After all, she still had an entire life to do it all again.