What is the transient bleed valve in a gas turbine engine?
The high pressure compressor in a typical engine has many stages (e.g. GE90-115 has 9) all running on the same shaft, and thus all at the same speed. The problem is that sometimes the flow/pressure that the front stages are producing does not match the flow/pressure that the back stages are able to accept. When this happens, the compressor will stall (this is bad).

This can happen in several different situations, and one of them is a quick transient.  For example a very quick acceleration from idle to full power.

During this quick transient, the front stages might be trying to pump too much air versus what the back stages are able to accept. Eventually, everything will even out, but for a few seconds there may be a mismatch.

There are several different ways that engine designers can accommodate this situation.  One is a transient bleed valve.

When the engine performs a very quick acceleration, the transient bleed valve is opened and some of the air from the compressor is just dumped overboard.  When the engine reaches a steady operating condition, the valve is closed.