Look at the Vmbe graph fig 4.7. If I had calculated from the speed tables in fig 4.8/9 a V1 of say 158 knots but after going through the Vmbe limit graph that limit was 150 kts I would have to reduce the take -off mass by 8 x 300kgs = 2400kgs to make V1 equal the Vmbe limit. (see notes at bottom of graph)

Remember V1 cannot be higher than the Vmbe limit because we couldn't stop in the event of a rejected take-off.

Now if the runway has an up-slope it will help us to stop and the notes say that I can increase the Vmbe limit by 2 kts for each each 1% of upslope.

As each kt is equal to 300kgs this increases the allowable take off mass.