I just looked up the "rules"
FIRST CHECKED BAG FREE FOR MILEAGEPLUS EXPLORER AND ONEPASS PLUS CARD MEMBERS
Primary cardmembers and one companion travelling on the same reservation will receive their first standard checked bag free each time they fly United when they purchase their tickets with their MileagePlus Explorer or OnePass Plus Card (a savings of up to $100 per round trip).
Terms and Conditions
Eligible credit cardmembers will receive their applicable standard checked bag free on flights operated by United; codeshare flights are not eligible. The primary cardmember's MileagePlus account number must be listed in their reservation and they must pay for their tickets with their qualifying United MileagePlus Chase Credit Card.
One companion traveling on the same reservation with the eligible primary credit Cardmember will also receive their applicable checked bag free.
Checked bags must meet weight and size restrictions or will be subject to additional fees. Learn more about our baggage policy.
No refunds or credits will be issued after travel has been completed.
Unpublished fares, such as priceline.com and Hotwire, are not eligible.
New United MileagePlus Chase primary credit card members will be eligible for their applicable free checked bag after their first use of their credit card.
Offer subject to change without notice. Other restrictions may apply.
If you have a question regarding whether your credit card is eligible for this benefit, please call the customer service number on the back of your card.