Inbreeding can be a horrible CURSE , or a marvelous BLESSING! It is an excellent tool to be used by conscientious breeders looking for "deeply rooted" recessives! Inbreeding brings these "hidden secrets" out of hiding, so they can be properly dealt with. The great stuff can then be "polished" and strengthened through tighter inbreeding on that particular mantis, the BAD stuff can be "washed-out" through identification & elimination! Inbreeding on "a species" without extensive knowledge of the mantis themselves, their nymph mates, etc is without a doubt the most dangerous form of breeding! It is much safer to just outcross, or "type" breed! That is the reason that most novices are advised NOT to inbreed!!! Without a long-term breeding program in mind, and the resources and dedication to accomplish it , all such breedings are in vain! Worse of all, others will most likely continue to breed such mantis without having sufficient knowledge about their faults & virtues, thereby compounding this problem even more! Inbreeding is a problem for all diploid animals. Diploid basically means you get a copy of every gene from each of your parents. For many of these genes you only require one working copy to exist and so the working one is said to be dominant over the broken one.
If one of your parents have a recessive copy (said to be carriers) than they have a 50% chance of passing that broken gene to their offspring but since the other parent probably has 2 working copies the child will get one working copy from the other parent.
If inbreeding happens there is a much higher chance that BOTH parents are carriers (have one broken copy) so they have a 25% chance of BOTH parents passing the broken gene to their offspring.
When you consider the vast number of genes in a human, animal or even insect it is very easy for at least a few important genes to be inherited broken from both parents if their is inbreeding.