Some ICO uses Dutch auction to distribute their token on pre-sale. There are main characteristics :

• A finite number of token is for sell.
• A reserve price (The price will not be lower than this value).
• Start price (The price at which the token is sold when the auction price).
• At the end, every participant pay their tokens at the same price.
• Each participant chooses the price and the number of token he wants.
• People who are willing to pay a higher amount will be given priority in the distribution of tokens.
• When all tokens are sold, the auction end and the token price is fixed. If people had place a lower bid they will not receive token (and they will not pay their bids)

Here is a video where dutch auction is well explained : 

So, if you want to participate to a Dutch auction there is 2 main strategies you can follow :

• Place a high bid to be sure to get token and let others reduce the price by their bids. When you think that the priority is to get token, you can chose to place a bid at the start price.
• Place a very low bid because you are not convinced that the token price will be high on the market. So you place a low bid but you take the risk to get no one.