A contact partner is someone who you have casual no-strings-attached sexual contact with to mutually give/receive instructions and feedback. This requires being completely non-judgemental and open to helping your partner with whatever anxieties and weaknesses they have. This is a way of improving your sexual performance, building communication skills, working out sexual anxieties, and developing a sensitivity to your partner's pleasure.
For people familiar with consent culture, getting explicit permission for everything and expressing honest and open intentions, being contact partners is not any harder than starting any other casual relationship. You just express that you want to help them get better as well as get better yourself.