For example If I am running with 1 loaded public key BSGS and for argument sake with 1 key loaded I am operation at 100 million keys per second but if I substract and load  1000 of the same key just offset by a different distance from from the target key my keys per second drops 100 million/1000 but now I have 1000 more possible chances of a collision because i have added 1000 targets.

Example
03b6c66d90910721ac8e6f8ef0ebb222fff638227122b21c5853e787ca3d119f08 # - 651321717934608777722865459537368854
030e7b9ccf0feabe79d8745a89b575b52ff21333368b4a9c95c72dbef88a6105e2 # + 651321717934608777722865459537368854
029a564b60e6bed3449052228a609334b43d59bc5f79ca5c622334658ce89b6026 # - 657967857913533357087384494838770577
03a127833050783f814ee013e796fbc937067a86741564bdb9086a6e6e6398e11d # + 657967857913533357087384494838770577
03d0203676b61edda6208b2f9b204de5e8ee2849723458dc7531b1eaba503de390 # - 664613997892457936451903530140172300
0247dd5a10ae3aab4187c7af2a5babcc8ee401ed09e67fbc6ea75e35cc8471d8c9 # + 664613997892457936451903530140172300
02ceb6cbbcdbdf5ef7150682150f4ce2c6f4807b349827dcdbdd1f2efa885a2630 # target


If one of these keys hits then i just add or subtract the decimal value on the right from the hex private key found and now I have the  key for 120 02ceb6cbbcdbdf5ef7150682150f4ce2c6f4807b349827dcdbdd1f2efa885a2630