For training, particularly for training to break a record, coaches will use a pace runner to set the pace (speed). Each lap will have a fresh pace runner who can maintain the target speed for the runner or group of runners being trained. As simon3142 mentioned, this is a pace bicycle.

A pace maker is someone who sets the pace in a race. This is intended to help others to go at the right speed. It might be used for training or in record attempts where it is important to go fast enough but no so fast that you run out of energy before the end. So a Pace bike would be a bicycle used for this purpose. In your case, one with an engine.

A little background will help. In bicycle racing or training there is often a rider who "sets the pace". That is, he determines how fast everyone goes .... they follow him at his speed. This 'lead' or 'pace' rider is providing training to those who are following him. In your example Metz created a special pace bicycle - one that has a motor ! This enabled him to set the speed or pace for those in the bike team that followed. With the motor attached to the bike it was much easier - he could always go faster/last longer and thus 'pace' or lead the bikes that followed him.