Hi Jo,
straight arm pushdown is a great exercise! Basically, what you do is stand in front of a triceps machine, facing the weights instead of having your back to them. Use a straight bar for this exercise. Grab the bar with an overhand grip and move back so that you can push the bar down with your arms straight - but try not to lock elbows. Stand straight up or, even better, lean slightly back, just take care not to lean forward when pushing the weight down as that defies the object. So you push the bar down to hip level and slowly let it back up till your hands are at shoulder level. All the while, keep back straight, shoulders down and abs in - you will notice that it really works your back and abs as well as your triceps.
Hope this is clear, let me know how you like it if you decide to try it out.
See ya,
Billy