Iso-lateral chest machines allow separate operation of the movement arms, allowing you to train your left and right sides independently almost like a "machine-version of dumbbells." When equally loaded, the iso-lateral design theoretically prevents a lifter’s “strong side” from taking the lion’s share of the press. Iso-lateral machines also allow the lifter to load a different amount of weight on the left and right side. This feature may be useful in special circumstances related to injury and rehabilitation.  A recent, particularly bothersome, trend is the use of alternating repetitions on iso-lateral chest machines. The lifter locks out one arm while performing a repetition with the other arm and see-saws back and forth throughout the set. While this alternating strategy may be useful if a lengthened isometric were held in the bottom position while the other arm performs a repetition, it's a waste of time to rest one arm in the locked out position. In general, iso-lateral machines are best used using the same technique as a machine chest press with a non-independent movement arm.