That the left photon moves towards the left and is polarised in the horizontal plane .
That the right photon moves towards the right and is polarised in a vertical plane .
That the angle of the left filter is zero.
What the sketch shows is, that when the angle "alpha" of the right filter
is 90 degrees that an entangled photon pair goes to both filters and will be counted twice and simultaneous
is 0 degrees that from an entangled photon pair only the left photon goes through a filter and will be counted once
is 45 degrees that from an entangled photon pair at least the left photon will be counted and that there is a 50% chance that also the right photon (will go through a filter) and will be counted