Flash into B main

Walk forward (hold Shift+W), and as you move, jump-throw: hold left click, then jump and let go at the same moment.

From B site. Stand against the wall at the top of the steps. Aim at the left edge of the pillar ahead.

Practice (with sv_cheats 1): setpos -1106.07 1288.33 4.21;setang 5.09 -113.82 0