I love your book cos its yours and with the others on this site, maybe we can one day write an 'addendum/errata' volume. BUT, the book makes us think.
Your question raises the ugly side of SCR's. "very difficult to turn off".
In control circuitry using SCR's where they are driven from DC supplies, the switching OFF of these monsters requires a thing called 'FORCED COMMUTATION'.
The ONLY way to turn them OFF is to force a reverse current through the thyristor to reduce the anode current to zero, AND, at the same time make very sure that the gate circuit is well and truly shutdown, locked off, shortcircuited, expunged, whatever; because when the SCR actually goes into its forward blocking state, the anode voltage will rise, at a very great rate, and this rise in forward voltage will allow parasitic currents to flow in the SCR which, MAY trigger the gate circuit into conduction.
Having a look at the data sheet for the BT151, it says, under 'characteristics';
"rate of rise of OFF state voltage that will not trigger any device" Rgk = open circuit; dV/dT<50 v/microsec.
AND for Rgk = 100 ohm; dV/dT <200 V/microsec.
These two values given for gate to cathode resistance plays into your previous question about the reverse diode. An SCR needs a LOT MORE than a diode to shut it off in forced commutation duty.
Now to your question.
When the pulse is applied to the transformer, the current does flow in the direction of the red arrow. The pulse transformer has its own secondary circuit comprising the winding upward through the SCR and through the capacitor (which is discharged because the SCR is conducting) and then back to the pulse transformer. When the anode current falls to less than the holding current, the SCR blocks and the anode voltage rises to Edc. The pulse current also stops flowing because the SCR is now open circuit.
You can see that the capacitor is required to return the current to the pulse transformer.
As an aside, in the diagram the load is shown as a resistor. BUT, frequently the load is an inductor or a motor, and iF there is a commutation failure, (the SCR doesnt switch off)then the inductive load looks like a short circuit and then the fuses blow and if its a big motor, even the street light go dim.
This explains the beauty of IGBT's. (SCR's that can be turned OFF)
Hope this helps.