To help identify what is paper-base and what is a tone, cover a long edge of the test-strip with the arm of the easel - it will give a direct comparison for each stage of the strip.

On multi-contrast paper, it is also possible to flash with grade-00 or grade-5, instead of white light. That can make a handy difference, as (for example) over-exposed clouds may be relatively low contrast within themselves.

Like everyone else has mentioned, this 'adds' the ineffective exposure which would otherwise be at the bottom left of the log-exposure/density curve, running pretty much along the x-axis. And no, it isn't an everyday technique unless you have very unusual negatives. EDIT: It could become more standard for you if you shoot contrasty scenes on Ilford Direct Positive paper though.