I think i'll be able to help you.
I'll take Bolg to explain this.
When bolg can wound, look what roll you should make to kill.
So def 4/5 need a 4+ and 6/7 need 5+. Now that Bolg has a 2h, the wounding becomes for d4/5 3+ and for 6/7 a 4+. That easy.
Serpent guards themselfs do not have a rule like that, the riders do (sorry I am a nitpicker).
When you charge you get the 1+ bonus, with s3 4/5 need a 5, where 6/7 need a 6. (just lower it when you charge, when you are charged you lose this bonus).
Now to let you in om somthing intresting: These +1 bonusus stack. So lets take the goblin prowler. With a 2h in his hand and his backstab rule, you get 2+ on your wounds. That would be forr d4/5 3+ and 6/7 4+.
To make it even more intresting, if you would use an easterling warpriest to give your prowler a s6 attack, you have an assured kill on def 4-, because you would need a 1 to kill them. Otherwise a 2+, what is really likely to happen.
Hope that I helped you
Edit: I forgot the piercing strike rule
, add this and with a good role you can inflict 2 wounds on almost every opponent