Hydro is hollow and contains some kind of lubricant (which doesn't lubricate the elastic externally - ie. against the inner pole wall). The idea is that, as the elastic stretches, the fluid is compressed and acts hydraulically to increase the pressure you can apply to a running fish, increasing the more the elastic stretches. It can be a bit brutal in the heavier sizes, but it does last a long time and the lighter grades are versatile, beacuse they stretch easily initially but apply more pressure the further it stretches. It's also more difficult to "bottom out" - there always seems to be a bit more in reserve. I prefer latex in heavier grades. It has more stretch and seems to recover faster.