A cycle can be defined a series of events that happen in a regular pattern so we could say that all 3 legs of a triathlon are actually cycles.
Every swim starts with a singular moment. A push off from the wall, a deep water start or a run in from shore. After that start..or ignition it is then a cycle of movements. So once you are flat and extended out in the water, what is the first movement of the swim cycle?
If you have got yourself on the surface of the water. The first move is the catch...Next is what is commonly and I think potentially erroneously referred to as the pull...but while both of those start to happen..there is an equal and opposite sequence going on with the opposite arm. The link to both sides is your rotation or body roll. I shall now put the order list wise
Reach
Catch
Roll..and pull
Roll..and push
Recover
Hand entry..and reach..but you can only get full reach if you Roll.
So this piece is mainly getting us focused on the rotation. The reach and roll if you will. The favoured approach is to be hip driven rather than just shoulder driven. Hips and shoulders are connected through the core..so if you can breathe..reach and roll baby! Oh and if you don't Roll and swim flat..you are restricted on how high you can lift the arm out of the water. Try lying on the floor flat and see how high you can get the arm up. Now try it rolling onto your side.