[Tinyos-help] Acknowledge frames may be manually transmitted

[Salvo Fiduccia]

Hi all,

at page 42 of CC2420 datasheet 
(http://focus.ti.com/lit/ds/symlink/cc2420.pdf)
there is: "Acknowledge frames may be manually transmitted using normal 
data transmission if desired."
What this mean?
I have a sensor sending messages to a basestation/host. I would send an ack (from basestation to node) only if a condition on received packets is verified. Could it be possible to send an ack like a message from the basestation?

Thanks for any help