What is the zone that disappears when actin filaments slide?
At the centre of contractile unit, sarcomere, there is H zone. It is also at the centre of dark band/A band. The H zone disappears when actin filaments slide inside A band during contraction of myofibril.
The illustration above is that of sarcomere. It is delimited by Z lines on both sides. There is dark A band where myofilaments myosin and actin overlap, but at its centre in relaxed condition a light area could be seen known as H band. When actin filaments slide inside A band during contraction, H zone is naturally obliterated.