Take the scenario as follows: 
Bid increment is $30. One user has an autobid of $372 and the other user places a bid of $355. The autobid comes into play so the current bid is now $372 with the autobidder. The next bid will then be $402.

So generally speaking, the autobid kicks in regardless of whether it is less than the next auto-increment or not. And the next bid is the autobid + increment.

Here’s another example: 
Bid increment is $5. UserA has the autobid of $52, UserB bids $50. The current bid jumps up to $52. The next bid for all users is $57 (i.e. autobid + increment).

When a user tries to place a maxbid that is lower than the next increment, they will get the following error:

Unable to place bid. It is not higher than the current highest bid plus the bid increment. Please check the price and make a fresh bid.