"It made a big difference to my match stamina. I couldn't imagine I would have been so energetic during the match - it really gave me a welcomed extra boost!"

Vladimir Kramnik is referring to the favorable result that a certain activity had on his match endurance. He was amazed by the amount of energy he had throughout the match, and credits the activity for providing him an additional increase. This recommends that the activity was useful to his physical and mental wellbeing, enabling him to carry out much better during the match. It is likely that the activity was something that enhanced his physical conditioning, such as running or weight training, or something that enhanced his mental focus, such as meditation or visualization. Whatever the activity was, it plainly had a positive effect on Kramnik's performance, and he is grateful for the extra boost it offered him.