It doesn't actually make the rangefinder patch brighter. It just blocks out part of the primary image so the secondary image is more visible.
(In effect, you're turning your camera into a Leningrad or a Werra: these two cameras have a rangefinder spot that's completely reflective, so the primary and secondary images don't mix.)
However, what you give up with this is the possibility of coincident-image focusing: aligning two semi-transparent images until they merge. All you've got left is split-image focusing: aligning vertical edges that extend through the rangefinder patch.
Split-image focusing is much more positive and capable of higher accuracy -- IF there are well-defined vertical lines in the subject to use. If there aren't, you can't focus at all. A well-adjusted coincident image rangefinder can focus on such tricky subjects as fabric textures, fields of grass or wheat, or a person's hair, while a split-image rangefinder will be stumped by such subjects.
Of course, using the tape trick, you're not completely masking off the primary image as accurately as it's done on, say, a Leningrad, in which the secondary image is a sharply-defined rectangle. With this technique, you'll still have a blurry boundary between the primary and secondary images in which coincident-image focusing is possible (sorta.) Anybody who's used an Argus C3 will recognize the effect immediately: the C3 had a "split-image" rangefinder but no optics to focus the split, so you'd have a primary image in the lower half of the view, a secondary image in the upper half, and a blurry boundary in which they sort of mixed.
I'm not sure that turning your camera's RF into the equivalent of a C3's is a step forward, but if you don't find it helpful you can always peel off the tape! Just remember that while you're gaining RF-spot clarity, you're giving up "focusability" under some conditions.
