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Superpro's polyurethane bushing have knurling on the inside diameter to help reduce contact area and retain lubrication for lower friction, longer life and to reduce squeaking.
The Superpro urethane bushings for the trailing arm have these grooves around the outside of the bushing to allow for a slight amount of flex to reduce bind and to retain lubricant.
The trailing arm bushings that are in the rear spindle also have similar grooves. Multilink suspensions often require bushing movement in three degrees of freedom so some flex is needed to prevent binding. This is an issue that many bushing companies don't understand and by making solid bushings they create a situation where the suspension binds under compression which can result in both a poor ride and a progression to oversteer in hard cornering which can be sudden and hard to control.
Howard Watanabe pushes out the bushings of the trailing arm using a hydraulic press. 
The other bushing is also pressed out of the spindle. 
The new bushings are greased up with the included silicone grease. Silicone grease is waterproof and very tenacious to help prevent squeaking for a long time.
The new bushings are easily pressed into the trailing arm by hand. 
