Dyna-Sys is the next generation MTB 10-speed drivetrain system from Shimano. The new Deore XT 10-speed cassette uses the Dyna-Sys system.
Dyna-Sys provides more efficient power transfer. The power of the rider is transferred more efficiently due to reduced chain tension, less need to change gear and improved continuous cadence due to a closer chainring combination on the chainset and wider cassette ratios.
These points improve pedalling efficiency, give smoother and more comfortable front shifting while having less affect on pedal feed back when used on suspension bikes.