The principle of deletion mapping involves crossing a strain which has a point mutation in a gene, with multiple strains who each carry a deletion in a different region of the same gene.
Wherever recombination occurs between the two strains to produce a wild-type (+) gene (regardless of frequency), the point mutation cannot lie within the region of the deletion.
If recombination cannot produce any wild-type genes, then it is reasonable to conclude that the point mutation and deletion are found within the same stretch of DNA.
In other words, there is a feasible recombination possibility between the point mutant and del-2 in which a length of DNA could be made that contained neither the point mutation, nor the deletion, indicating that the mutations in these two strains cannot be in the same region.
Note that not all crossovers between the point mutant and del-2 will yield (+) recombinants; in this case only those crossover events that occur between the point mutant and the 5' end of the deletion would inherit the wild-type sequence.