Pre-exercise hyperhydration delays dehydration and improves endurance capacity during 2 h of cycling in a temperate climate.
This study demonstrated that drinking water with glycerol prior to endurance exercise "improved peak power output and time to exhaustion by 5 and 14%, respectively." (glycerol helps the body retain water)
Note: During pre-exercise hyperhydration subjects ingested 1776 + or - 102 ml of fluid, together with 82.0 + or - 4.7 g of glycerol. That's 60 oz (just under 1/2 gallon!) given during 80 minutes pre exercise- who does that?? I know what you are thinking- One subject out six total had to urinate during the hyperhydration trial.
Authors conclude that pre-exercise hyperhydration improves endurance capacity and peak power output and decreases heart rate and thirst sensation, but does not reduce rectal temperature during 2 h of moderate to intense cycling in a moderate environment when fluid consumption is 33% of sweat losses
Another good question - what happens when you hyperhydrate without glycerol?
Journal of PHYSIOLOGICAL ANTHROPOLOGY Vol. 27 (2008) , No. 5 pp.263-271