This is an active-High signal. When powerdown is asserted, the transceivers in the Aurora 8B/10B core are turned off, putting them into a non-operating, low-power mode. When powerdown is deasserted, the core automatically resets. gt_reset must be asserted after powerdown deassertion as indicated in the guidelines of the transceiver user guide.
CAUTION! Use care when asserting the powerdown signal on cores that use tx_out_clk (see Serial Transceiver Reference Clock Interface). tx_out_clk stops when the GTP, GTX, and GTH transceivers are powered down. See the 7 Series FPGAs GTX/GTH Transceivers User Guide (UG476) [Ref 3], 7 Series FPGAs GTP Transceivers User Guide (ug482) [Ref 2], and the UltraScale Architecture GTH Transceivers User Guide (UG576) [Ref 1].