It is in their terms and conditions
Boost Your Deposit By 5%Terms & Conditions
1. You must opt-in to receive the 5% boost on your Bitcoin deposits.
2. The opt-in toggle is placed in the deposit screen, below the amount field.
3. The boosted deposit amount is subject to a 10x (10-times) rollover requirement before any funds or winnings can be withdrawn.
4. Non-boosted deposits will be subject to the regular 1x (one-time) rollover requirement.
5. The 5% boost is automatically added to your cash balance and not as a free play bonus.
6. Rollover requirements for additional promo codes must be met if used with boosted deposits
7. This is a limited time offer.
8. Please see LowVig’s General Rules for additional terms and conditions, which apply to any and all promotions.