Signaling is per block, not each transaction. Miners create blocks with a specific version bit set. Bit 4 is a signal that that miner likes the idea of BIP91. Miners can set it or not. 80+% recently have.
Miners and users/nodes (you, me, coinbase, bitstamp, etc.) can accept blocks with bit 4 or reject them. Enforcement would be rejection of blocks which don't fit agreed upon condition after certain threshold (80% over 336 blocks) is met. But very few miners or users/nodes actually run a client which has code to enforce BIP91, so it will not be enforced automatically. With only 239 blocks left to activate BIP91 it's extremely unlikely that supermajority of nodes/users will upgrade to enforcing client. BIP91 getting locked in and activated would probably not have any effect in practice, but it is indicative of desires of bitcoin miners in general.
BIP141 idea is activation of segwit. Miners can signal they agree with it by setting bit 1 in block version. Bitcoin Core client is used by a good portion of miners and users/nodes and bitcoin Core does have actual code in it to enforce bit 1. There is much steeper threshold (95% over a year) before enforcement begins. That's meant to get supermajority all upgraded and ready to go in time. The 5% will be out of luck but hopefully they'll wake up and upgrade once their stuff starts failing. BIP141 is tracked here http://segwit.co/#