For those who don't know yet, Spellblades (Players Guide to Faerun) are an enchantment that can be added to any weapon for 6k gold. Spellblades are tuned to a particular spell (or power if you use full transparency), and when you are targetted by that spell/power, the blade absorbs it, and you can redirect the effect next round as a free action.
Lets say I make 2 spellblade weapons tuned to Spell X (X can be any targetted spell). I carry one, and give the other to my psicrystal/familiar/any other intelligent and loyal minion which can follow reasonably complicated instructions.
At the start of the day, I could cast spell X on my minion. It would be absorbed by its spellblade, and then on its next round, as a free action, my minion could redirect Spell X's effects to me. Which would be absorbed by my spellblade. And on my next round I redirect it back at it and so on and so forth. In this manner we can bounce Spell X between one another, every round, as free actions. Spelblades do not state that there is any limit to the number of spells it can absorb in one round, so I could spam spell X, and keep all of it ricocheting between me and my minion (hence the name "Spellblade Tennis").
Upon seeing an enemy, We can stop redirecting spell X at each other, and start channeling it towards the enemy, one spell at a time, as free actions, until said enemy drops dead (assuming spell X can kill the enemy at all, so it might work best if its a "sure-kill" spell like searing orbs of fire. I imagine Tippy's Cindy must be dancing with joy by now).
It is possible to sustain Spellblade Tennis through the night, and build up on it further the next morning, literally letting you accumulate thousands or millions of spells. You need at least 2 individuals with spellblades to sustain it at any one point, so either have some people work in shifts, or get (at least 2) minions that don't need sleep (possibly including yourself).
Of course all this relies on a strict RAW of spellblades. By RAI, it might be argued that since spellblades are enchanted with spellturning, they might cause a resonating field when used in such a manner, rather than bouncing around indefinitely.
Charging the Spell Battery
In Essence, Spellblade Tennis provides a means for you to build up spells for later use. Because the spell battery can be transferred between pairs of Spellblades, you don't necessarily have to be present during the charging up process. You could say create a secure location (such as a time accelerated demiplane), and park 2 sleepless minions with spellblades, and a self-resetting trap that churns out the key spell every round. When you need a lot firepower, you can go to this location, order your minions to transfer the spell battery to you, and then go and blow something up.
-Whoops, its Players Guide to Faerun, not Magic of Faerun.
-Added note on sustaining through the night in shifts. Thanks to Lysander
-Added Demiplane Shenanigans. Thanks to Brock Samson