When CRTC HSYNC is active, the Gate Array immediately outputs the palette colour black. If the HSYNC is set to 14 characters then black will be output for 14µs. The HSYNC pulse width needs to be at least 2µs for Gate Array to change the video mode.
The HSYNC is modified before being sent to the monitor. It only happens after 2µs after the HSYNC from the CRTC and stay a maximum of 4µs.
For example, if CRTC R2=46, and CRTC HSYNC width is 14 chars then monitor hsync starts at 48 and lasts only until 51 included.