Every eligible trade made through the Balancer Exchange Proxy results in some BAL being allocated to the address (EOAs only) that sent the transaction. An eligible trade is a trade containing one or more eligible swaps, where an eligible swap is one between any two tokens on the whitelist. Claims are made available at the BAL claims interface on Wednesday (UTC time) following the end (00:00 UTC Monday) of the weekly period in which the trade occurred. You need to CLAIM your BAL.
The amount of BAL awarded to a trade is a function of the number of eligible swaps in the trade (N), which determines a number of gas units (G), the median gas price  of the block the transaction was included in (M), and the BAL/ETH price provided by the CoinGecko closest to the block time (P).
The amount of BAL to be received by the user for a trade is computed as
Because this program only partially covers gas costs (with very specific caps to prevent attempts at gaming the system), and doesn’t cover trading fees, it does not incentivize wash trading or trading any more than a user was initially planning. Estimates are provided on the UI, but actual values are computed off-chain weekly using an open source script, so that any suspicious activity can be filtered out.