With a DLM reset your connection is, as the name suggests, reset. I.e you will experience a short interruption to the service.
A DLM reset takes you back to the start like a brand new connection this would be probably happen after engineer visit and a fault fixed
As said it basically resets your Broadband back to day 1, a bit like resetting your phone back to its factory settings.
Depending on what type of DSLAM you’re on it can have a temporary negative impact on your sync speed because a DLM will switch off G.INP and switching it back on is an automated process done by the DSLAM. Openreach Engineers have no control over it.
I somehow suspect the OP is confusing cause and effect and is not talking about a DLM reset at all but rather DLM taking action on the fault which is causing his problems.
Ah right, yeah if the DLM see a high error count and drops in sync then it will keep lowering the sync rate to try to stabilise the connection.
Openreach Engineers see the information as something called Brandenburg Data. They get a basic overview of it on the job but can get more in depth info on their Laptop. It also shows the data of other DSL Circuits on the same DP so they can narrow down if the fault is unique to a particular line or if it’s affecting others.